When we talk about an invasion, imagine a big group of ants marching into your favorite sandbox without asking. They just come in and start playing with your toys, even though you didn’t invite them. This is like when someone enters a place where they are not supposed to be, without permission, and starts doing things there.
Another way to think about an invasion is like a surprise party, but not the fun kind. It’s like if you were having a quiet time in your room and suddenly a bunch of people you don’t know burst in and start rearranging your stuff and playing with your toys. It can feel scary and confusing because you didn’t ask them to come in, and they are doing things you don’t want them to do.
In nature, an invasion is like when a plant or animal comes to a new place and starts living there, even though it doesn’t belong there. It might take over the area, pushing out the plants and animals that were there first. This can change the whole environment, like when a new kid comes to the playground and takes over all the swings, leaving none for the other kids.