Gravitational waves are ripples or waves in the fabric of spacetime. Space is not empty, it is an invisible “fabric.” As objects move through this space fabric (i.e. stars or planets), they can affect it by causing waves. The more massive an object is, the more it affects and the bigger its waves are.
Imagine dropping two beach balls into a pool of still water. If you drop a small beach ball into the pool, it causes small waves that disappear quickly. Now, if you drop a large ball in, its waves will be much bigger and spread much further out in the water. This is similar to the gravitational waves created by objects in space.