Okay, kiddo. Have you ever played a game of hide-and-seek with your friends where you agree to certain rules, like how long you'll count and where you can hide? Well, grown-up countries play a similar game called the Treaty on Open Skies!
It's a set of rules that some countries agreed to, which says that they can fly planes over each other's territory and take pictures of what's on the ground. Just like how you can't hide in certain places during hide-and-seek, there are rules about what kind of planes they can use and where they can fly.
This helps the countries know what each other are up to and builds trust between them. But some countries don't want to play this game, so not all countries are part of the treaty. And just like how the grown-ups have a "home base" in hide-and-seek, these countries have certain places they can't be flown over, like their military bases.
So that's the Treaty on Open Skies! It's like a game of hide-and-seek for grown-up countries, except they fly planes and take pictures instead of hiding and seeking.