To put it simply, baseball tickets in this movie symbolize selfishness and a total lack of respect for the lives of others.
So, yeah, you're probably wondering how we got to this conclusion. But don't worry; we're not telling you you're a terrible person if you recently went to a ball game. Allow us to explain.
When Juror #7 enters the jury room, one of the first things he says is, "Okay, men. Let's take our seats, huh? We can all get out of here pretty quick. I have tickets to that ball game tonight. Yanks and Cleveland. We've got this kid Modjelewski in there." So at this point, we're already thinking: Yeah, this guy is more interested in a ball game than in reaching a fair verdict. Even when he agrees to take more time to deliberate, the dude says, "Supposin' we take five minutes? So what? Let's take an hour. The ball game doesn't start till eight o'clock."
Well, maybe that's not so terrible—but what this guy reveals later sure is. Toward the end of the movie, we find out that this dude has been voting for whichever verdict he thinks will get him out of the jury room sooner. He doesn't even care about the outcome. At this point, Juror #11 rightfully jumps up to say, "What kind of a man are you? You have sat here and voted guilty with everyone else because there are some baseball tickets burning a hole in your pocket."
So there you have it: baseball tickets in this case represent the fact that this guy cares more about getting to a ballgame than he does about the life and death of the kid who's been put on trial. It'd be nice to think that no one could ever be this selfish in real life, but who knows? Chances are you'll find someone like this somewhere.