You have two sides, one out in the field and one in.

Each man that's in the side that's in goes out and when he's out he comes in and the next man goes in until he's 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 those 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.