Annexation is when one place takes over another place and becomes part of it. It's like when you invite a friend to play at your house and they become part of your game.
But for cities and countries, annexation is a more official process that requires a lot of paperwork and planning. Let's say you live in a small town and the big city nearby wants to annex your town. That means the big city wants to become responsible for making rules and taking care of the people who live in your town.
But not everyone might like this idea. Some people might want to keep their small town the way it is instead of being part of a bigger city. So before anything can happen, there needs to be a lot of discussions and meetings to figure out what's best for everyone.
Once everyone has agreed, the big city will start taking care of things like fixing roads, providing police and fire services, and making rules for the town. The people who used to run the small town might still have a say in some things, but now they have to work with the big city to make decisions. And that's how annexation works!