Okay kiddo, so you know how the earth has a lot of hot stuff deep down inside, right? Well sometimes, this hot stuff called magma comes out of the earth through a crack or hole. When this happens in the middle of the ocean, it creates a volcanic island!
Imagine you're playing with play-dough and you push something up from the middle to make a hill. That's kind of like what happens when the magma comes out of the earth and starts building up in the ocean. Over time, more and more magma comes out and the hill gets bigger and bigger until it's sticking out of the water and becomes an island!
Volcanic islands can be really exciting because they're formed by explosions and lava flows. As time goes on, new rocks and land formations can form on the island, and plants and animals can move in and make it their home. But, since volcanic islands are formed by lava and magma, they can also be dangerous places to be around when there's an eruption.
So, that's what a volcanic island is, an island formed by lots of magma coming out of the earth and building up until it's a big, rocky island in the middle of the ocean!