In the United States, in organized baseball, baseball teams are grouped into leagues. Without trying to explain the system which I don't understand too well myself, there are two "major leagues" (informally, the "big leagues.") But there are also "minor leagues," which are not as good, and play against other minor league teams. Often, the minor league teams are used as training, a kind of apprenticeship, for players who hope to play on the big-league teams.
For example, the major league team in Boston is the Boston Red Sox. But there is also a minor league time, the Red Sox' "farm team," the Pawtucket Red Sox. They play good baseball, the tickets are cheap, and families that wouldn't want to spend the money or the time to go to a Boston Red Sox Game might go to a PawSox game. It's not great baseball but it's good baseball. On a nice summer weekend, maybe someone says "how about going to the PawSox game tonight?"
"He's out of your league" means "you're a minor league player. He's a major league player. He's so much better than you that he's in a different league."