In chapter 3, young Walter Cunningham has been invited to a meal after Jem found his younger sister beating him up over a conflict with their teacher. Scout is already in a foul mood, exasperated with Miss Caroline for not understanding the ways of Maycomb more quickly.

When Walter sits down for the meal, he requests molasses. Because his father doesn't have the financial resources that the Finch family possess, molasses is likely a treat that isn't readily available at the Cunningham residence. Thus, Walter pours it over everything, even his vegetables, which horrifies Scout. She asks him what the "sam hill" he's doing, and Calpurnia pulls her into the kitchen.

Calpurnia, who often stands in as a maternal figure for the Finch children, reminds Scout that Walter is her guest. This likely irks Scout for two reasons. First, she hadn't invited Walter there in the first place. She was rubbing his nose in the dirt when her brother had intervened and invited Walter to their home. Scout's rebuttal to Calpurnia's classification as their "guest" provides further insight into her character at this early point in the novel. She retorts that Walter is "just a Cunningham."

As a lawyer's daughter, Scout is afforded a better lifestyle than some other people in town. Because of her last name and family ties to Maycomb, she is treated with more respect than some other families. She is just a young child, but she has already begun to absorb the class structure in Maycomb, as revealed by this comment. Whether listening to the way Miss Caroline treats students with "cooties" or learning social etiquette from Aunt Alexandra, Scout has already developed a perspective about the advantages of being a Finch—and the disadvantages of being a Cunningham.

Calpurnia quickly admonishes Scout for her skewed perception as it does not reflect the views of Atticus. She tells Scout, "Anybody sets foot in
this house's yo' comp'ny, and don't you let me catch you remarkin' on their ways like you was so high and mighty!" Because Atticus leads through example and invests a great deal of time into meaningful conversations with his children, never shying away from difficult topics, Scout learns to value people because of their character, not their financial status.

Scout embarrasses Walter Cunningham Jr. during lunch by commenting on his strange eating habits in front of her family. Calpurnia immediately reprimands Scout in the kitchen and tells her that she isn't called to contradict anybody because of the way they eat. Calpurnia attempts to teach Scout the importance of treating her company with respect but Scout replies by saying, "He ain't company, Cal, he's just a Cunningham." Calpurnia responds by saying,

Don't matter who they are, anybody sets foot in this house's yo' comp'ny, and don't you let me catch you remarkin' on their ways like you was so high and mighty! Yo' folks might be better'n the Cunninghams but it don’t count for nothin' the way you're disgracin' 'em.

Scout's comment reveals her prejudice towards lower-class individuals. Evidently, Scout views Walter Cunningham Jr. as beneath her because he comes from a poor farming family in Old Sarum. Scout also knows Walter's father cannot afford to pay for Atticus's services and must barter using homegrown products. She is also aware that Walter cannot pay back the quarter Miss Caroline was willing to lend him.

Even though Scout is young and naïve, she is very impressionable. Scout has picked up the idea that she is better than certain children her age because her father is employed and she lives in town. She is unaware that she exercising classism and discriminating against Walter.

As Scout matures, she receives numerous valuable life lessons and learns the importance of equality, tolerance, and humanity. Atticus teaches Scout not to judge others based on race or social status. Later in the story, Scout's outlook on Walter Cunningham Jr. changes, and she acknowledges that he is not much different than her. Scout even comes to Walter's defense when Aunt Alexandra calls him trash.

In a novel that is largely about the falseness of prejudice, so to speak, Scout's views of Walter Cunningham (and, by extension, his family) are highly significant. 

Saying the Walter does not qualify as "company," Scout is implying that the popular view of the Cunningham's as nothing more than poor may be expanded to also suggest that the family should not be counted and do not belong in the category of "real people."

Scout's statement is an example of "classism" or class discrimination. Calpurnia's response is that Scout should not put herself (or her humanity) above that of others. 

This opposed pair of view points resonates with many other episodes of prejudice being challenged in the novel. Boo Radley is pre-judged by many in the community, but turns out to be quite a different person from the prevalent, gossip-driven view of his character.

Thus the novel's examination of race prejudice stands alongside its treatment of class prejudice, age prejudice and gossip-based local prejudice.

The notion of empathy and subjectivity of perspective is repeated throughout the novel and addressed directly on several occasions.

The lesson Atticus gives to Scout can be said to stand as a unifying thread that connects the various episodes of the novel and which functions also as the moral of the book. 

“First of all,” he said, “if you can learn a simple trick, Scout, you’ll get along a lot better with all kinds of folks. You never really understand a person until you consider things from his point of view—“

“Sir?”

“—until you climb into his skin and walk around in it.”

Scout says this to Calpurnia in Chapter 3.  She is talking about Walter Cunningham.

Walter comes to eat at the Finch home because Jem invites him.  Jem feels bad, I think, because Scout had been beating Walter up.

When Scout says this, she is telling Calpurnia that Walter does not deserve to be treated like company.  She says that he is just a Cunningham and is therefore low class and does not need to be treated well.

Calpurnia gets really angry at her and tells her that anyone who comes in the house is company whether they are rich or poor.

"Hush your mouth! Don't matter who they are, anybody sets foot in
this house's yo' comp'ny, and don't you let me catch you remarkin'
on their ways like you was so high and mighty!

It seems rather out of character for Scout to be disdainful of Walter Cunningham, whom she has defended to Miss Caroline at school.  By saying "He ain't company, Cal; he's just a Cunningham," Scout seems to be implying that Walter is no one special since his father and he have come to the house to give payment to her father at times, and since she sees him all the time at school.  To Scout, "company" is someone who does not normally come over to the Finch house, such as the ladies of the Missionary Circle that Aunt Alexandra invites.  For this company, Scout has to dress up, as well.  Scout feels that the situation is too relaxed with Walter,and it is all right to criticize him about his excessive pouring of the syrup.

However, Calpurnia knows that doing so is not good manners.  When she calls Walter "company" she implies a different meaning: the boy is the guest of Scout and, therefore, she should make no remarks about his eating habits. As punishment, she sends Scout to the kitchen, where the servants ate in the old days.  Humiliated, Scout is grateful that she does not have to face Walter and Jem again.

What Scout means by this is that she thinks that Walter Cunningham is not as good as the Finches.  She thinks he is not worthy of being treated like a guest (of being honored in the Finch home).

Calpurnia does not stand for that at all.  She gets very angry at Scout.  She scolds her for being all high and mighty and thinking she is better than other people.  She gives Scout a "stinging smack" and tells her she should come eat in the kitchen because she doesn't deserve to eat with the others.

Scout feels that the Cunninghams are a class below her, and "company" is a fancy word referring to honored guests that are to be treated with respect and politeness.  Whenever parents usually refer to company, it is in the sense of "We've got company coming, so you'd better behave yourself!"  All etiquette, manners, dress, and behavior are supposed to be at their best in the presence of company.  But Walter is just a poor, dirty, little kid, and a Cunningham to boot.  Scout doesn't think that she should have to behave like his is "official" company, because she looks down on him a bit, pities him, and he's just a poor kid that they invited over for lunch.

Because of her attitude, she is a bit rude to poor Walter as he drowns his food in syrup.  Scout is tactlessly commenting on the syrup situation when Cal hauls her out of the room and tries to teach her a lesson in politeness;  Scout doesn't quite get it, and hence her remark.  But, she obeys anyway.