Okay kiddo, an island is like a big piece of land that is completely surrounded by water. You know how sometimes when you take a bath, the water gets close to the rim of the tub but it doesn't overflow onto the floor because there's a little space between the water and the top of the tub? Well, an island is like the tub, and the water is like the ocean all around it.
Some islands are really big, like Australia or Greenland, which are bigger than some countries! And some are really small, like a little patch of sand with a few palm trees on it. The animals and plants that live on an island are often different from those on the mainland because they have to adapt to living in a more isolated environment. And because there's no easy way to get off an island, sometimes people and animals can get stuck there for a long time.
Overall, an island is like a little world of its own that's floating in the middle of the ocean. Isn't that cool?