It's a baseball term, yes.
There are nine innings in a game, and the home team bats last, so the bottom of the ninth means that it's the last chance the home team will have to score the winning run.
There are three outs in an inning. If there are already two outs, then there is only one more chance to score the run -- or more accurately, if one more mistake is made, there will be no more chances.
So Michael is saying that this is their last chance - they have one more "at bat" (chance to score) to win the game.