Game design is when someone comes up with an idea for a game and figures out all the rules and things you can do in the game. It's like when you make up a game with your friends and decide what you need to do to win and how to play.
A game designer thinks about what the game should look like, what characters you can play as, and what you need to do to beat the game. They also think about how challenging the game should be, so that it's not too easy and not too hard.
Game designers have to think about what makes a game fun and exciting for people to play. They might add cool graphics or music to make the game more interesting. They also have to make sure the game is fair, so that everyone has a chance to win.
Once the game designer has figured out all the details of the game, they work with other people like programmers and artists to actually create the game. They use special software to make the game come to life on a computer or other device.
So basically, game design is like making up your own game, but more complicated and with a lot of work to make it happen!