When a story is written in first person point of view, the information the reader receives is seen only through the eyes of the narrator. Our perceptions are based on those of the speaker, so our understanding of the characters, conflict and plot development are based on what we learn from the narrator.
However, with this said, we sometimes have to ask if we can depend upon what the writer is saying: is he/she reliable? This can sometimes only be understood based on how the story is resolved. For example, to understand this, refer to Edgar Allan Poe's short story, "The Cask of Amontillado."
The story is told from Montresor's point of view. At the beginning of the story, he tells the reader that Fortunato has insulted him beyond bearing, but we never receive details of this and can only take Montresor's word for it:
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
We are quickly made aware that Montresor has been planning his revenge but is sly, having given his victim no hint of his intent:
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will.
As the story moves on, the reader begins to wonder at the validity of Montresor's intent as he comments on his family's coat of arms—this foreshadows the story's end:
“I forget your arms.”
“And the motto?”
“Nemo me impune lacessit.”
The last line means "No one attacks me with impunity" or no one hurts me and gets away with it. This attitude makes us think again about Montresor's belief that Fortunato has insulted him. As the story progresses and the reader discovers the speaker's intent, we may well begin to question Montresor's sanity as he chains Fortunato within a room he has made, and walls him up inside.
...I soon uncovered a quantity of building stone and mortar. With these materials and with the aid of my trowel, I began vigorously to wall up the entrance of the niche.
Montresor is not a reliable narrator. We cannot believe what he says.
Harper Lee's To Kill a Mockingbird is told in the first person by Scout Finch, a major character in the story. We find that she is insightful, but young. She is honest and reliable to the extent that her maturity allows. We see Scout's maturity when she worries that she will not be able to read with her father after being chastised by her teacher for being too educated. She insightfully notes that we don't worry about losing things we take for granted:
Until I feared I would lose it, I never loved to read. One does not love breathing.
At the same time, we realize that we can only rely on Scout to a certain extent because of her youth. At one point she describes her father, Atticus, as only her young eyes can "see" him:
Atticus was feeble: he was nearly fifty.
However, first person point of view allows a character to consider some of life's biggest questions, thereby posing those same questions to the audience. We learn of emotions and inner-conflicts. This is often times the real value of reading a first-person account.
In addition, because we can only know what the narrator knows, we can at times be as surprised as the narrator when something unexpected takes place because the narrator's knowledge is limited—and therefore, so is ours.