A game for 3 or more people (most often children). One of the people is 'it', AKA Simon. The others must do what Simon tells them to do. There is a catch, of course. You must always listen for the magic phrase is "Simon says". If Simon says "Simon says jump", you jump (if you don't jump, you're out). But if Simon says "jump", you don't jump (if you do jump, you're out). If Simon doesn't preface his command with "Simon says", you don't do it.
In general, it's the spirit of the command, not the actions that matters; if Simon says "Simon says touch you toes", you only have to show you're trying to touch your toes. It's the ability to distinguish between valid and invalid demands, rather than physically ability, that matters here.
It is Simon's task to try to get everyone out as quickly as possible, and it is every one else's job to stay 'in' for as long as possible. The last of Simon's followers to stay in wins (although the game is not always played all the way through). Until you try it, you can't understand how hard this can be. It is advised that, for maximum enjoyment, you try it while you are still a child.
Some closely related games are Mother, May I? (AKA Captain May I?), Port and Starboard (AKA Ship Captain), and of course, Follow the Leader.