Okay kiddo, imagine you have a bunch of Legos, and you can stick them together to make something cool like a spaceship. If you try to pull them apart, they will stay together because they are bound or stuck together.
Water molecules can also be bound or stuck to other things, like dirt or minerals in the ground. This means that the water isn't free to move around, it's kinda like it's trapped.
Scientists call this "bound water" because it's bound to something else, and it can't move as easily as regular water can. And this bound water can be really important for things like growing plants, because even though the water isn't free to move around, the plants can still use it to help them grow.
So think of it like a bunch of Legos stuck together, or water that's trapped with something else, and you'll understand what bound water is!