World domination is when someone wants to control the whole entire planet. It's kind of like a game of make-believe, where one person wants to be the boss of everyone else.
Think of playing with your toys. You have your favorite toy and you might pretend it's in charge of all the other toys. And you might try to make all the other toys do what your favorite toy wants. That's kind of like world domination.
Some people in real life might think that they should be in charge of everything and everyone, and they might try to make that happen. They might try to get lots of power, like an evil magician or a superhero villain, so they can tell everyone else what to do, where to go, or what to think.
But wanting to rule the world can be very dangerous and cause problems for lots of people. It's important for everyone to work together peacefully and share the world instead of letting one person take control.