You have two sides, one that's out in the field and one that's in.
The side that's out in the field tries to get the side that's in out.
Each man in the side that's in goes out, and when he's out he comes in and the next man goes out.
When they are all out, the side that's out comes in, and the side that's been in goes out and tries to get the side coming in out.
Sometimes you get men still in and not out.
When both sides have been in and out, twice, including the not outs, that's the end of the game.