You are either with us, or against us

"Ok kiddo, imagine that you are in a group of friends, and there is a game that you all want to play. Now, some of your friends want to play a different game, but you want to play this game.

So, you say to your friends, 'Hey, let's play this game!' and your friends say, 'No, we want to play this other game instead.'

Then, you might say, 'Well, if you don't want to play this game with me, then you are against me and we can't play together.' This means that you and your friends can't work together on this game, and you have to find other friends who want to play the game you want to play."

The same thing can happen in bigger groups, like countries. If one country wants to do something, and another country doesn't want to do the same thing, they might say, "You are either with us, or against us." This means that the two countries can't work together on that thing, and they might not be friends anymore.