Simon Says is a game for three or more players (most often children). One of the people is "it" -- i.e., Simon. The others must do what Simon tells them to do. The catch is, the magic phrase is "Simon says." If Simon says "Simon says jump," you jump (if you don't jump, you're out). However, if Simon says simply "jump," without first saying "Simon says", you don't jump (if you do jump, you're out). In general, it's the spirit of the command, not the actions that matters; if Simon says "Simon says touch your 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 physical 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).