Most of the men on the jury are from "out in the woods," not Maycomb citizens with whom Scout or Jem are familiar. Atticus explains Maycomb citizens either don't care about the trial enough to be on the jury or are scared of getting involved in the whole trial, so they make up excuses to avoid being on the jury. As a result, when Scout watches the trial, she always just sees the jury as a sea of unknown faces.
Eventually, Scout and Jem learn from Atticus that one of the members of the jury is actually related to the Cunninghams. Specifically, he's a "double first cousin" of a Cunningham. If that sounds odd, it's because "two sisters married two brothers," as Atticus explains. Scout and Jem find this baffling.
What's important to note is that this particular jury member was the only one who wanted to rule that Tom was innocent, not guilty. It's ironic because the Cunninghams are the ones accusing Tom of the crime, but one of their relatives believes Tom is innocent and tries to defend him -- he was probably the reason the jury deliberated for a few hours instead of just a few minutes. Back when the jury was being assembled before the trial began, Atticus thought he'd recognized this jury member, and Atticus had the chance to strike him from the jury. He wisely chose not to do so.