It's quite simple really...

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 a man goes out to go in, the men who
are out try to get him out, and when he is out he goes
in and the next man in goes out and goes in. There are two
men called umpires who stay all out the time and they
decide when the men who are in are out. When both sides
have been in and all the men have been given out, and
both sides have been out twice after all the men have
been in, including those who are not out, that is the
end of the game!