This is a phrase used in baseball to refer to a pitcher or team whose pitching abilities and performance are significantly better than the opponents. It is used on a game-by-game basis, for example, "Joe really out-pitched the other team today." It is a slang phrase and is only a subjective description of the action on the field. There is no strict definition of being out-pitched, only that one team has a significant advantage. Veteran pitchers often out-pitch their rookie counterparts, but not always.
When a team is being out-pitched, they will typically change pitchers and try to change up the batting lineup in order to get back into the game. A good example of being outpitched is when a team gets shutout, meaning they did not score any runs in the game. In this example, the winning team's pitcher will have out-pitched his/her opponent. The phrase could also apply to segments of game, especially if a team changes strategy and is eventually able to overcome the skill of the pitcher and get hits and runs.