This statement is made by Atticus during his discussion with Sheriff Tate concerning the death of Bob Ewell. Atticus is still under the impression that Jem may have been responsible for Bob's death, and the sheriff has just told him to
"... hold on... Jem never stabbed Bob Ewell."
But Atticus is not thinking clearly, and he has not considered the possibility that Boo may have been responsible. (Or, he may possibly be considering allowing Jem to accept the blame instead.) Atticus believes that Heck may be trying to "hush up" Jem's involvement, and this is not acceptable to him. Atticus thanks Heck for this consideration, but tells him that
"I don't want my boy starting out with something like this over his head. Best way to clear the air is to have it all out in the open. Let the county come and bring sandwiches."
Atticus' statement simply suggests that he wants to keep everything out in the open--like inviting everyone in Maycomb to a picnic to announce the truth. Of course, Tate eventually sets Atticus straight--it was Boo who killed Bob--and enacts a coverup after all, but to protect Boo, not Jem.
The word "sandwiches" by itself does not have any particular significance in this statement, but used as a whole, the statement reinforces the idea that Atticus wants all the details about what happened that night to be out in the open. Atticus is under the impression that Jem killed Mr. Ewell in self-defense, and when Mr. Tate suggests that "Jem never stabbed Bob Ewell," Atticus thinks that he is proposing that what really happened be covered up, to protect Jem. Atticus, while expressing his appreciation to Mr. Tate for trying to save his son from the censure of the community, objects, saying that "Nobody's gonna hush anything up." He wants everything to be handled in an honest and straightforward manner, and uses the statement
"Let the county come and bring sandwiches"
to illustrate his point. The image of everybody coming out with their sandwich lunches, like a picnic, to sit back and hear and watch and see the truth play out presents an attitude of complete transparency. Instead of hiding what he believes is the truth, Atticus wants to be completely honest and let everyone learn, at their leisure, what really happened, in the belief that justice will prevail (Chapter 30).