The reference to Jem's playing football occurs in the opening of the novel. Scout says her brother's left arm had been broken "at the elbow" when he was twelve, which left it a bit shorter than his right arm. According to Scout, this did not bother Jem at all, once he realized he could continue to play football:
He couldn't have cared less, so long as he could pass and punt.
The implication is that Jem played the position of quarterback on the team.