In baseball games, it’s considered a momento to catch a fly ball (a ball which ends up being hit such that it goes into the crowd).

This is making a reference to that but with the Roman coliseum so, instead of a ball, the kid caught a decapitated gladiator’s head.

The joke is basically the inversion of expectations and, possibly, the gross out factor of contrasting the recreation of most sports with the violence of the coliseum.