Buoyancy is the lightness of an object in relationship to water. You see, all objects have weight as well as a size. Some objects, like a balloon, might be big but not weigh very much. Others might be small, but weigh a lot, like a rock. How big something is, compared to how much it weighs is known as density.
Water also has density.
Things that are put in the water might be more dense than the water itself. In that case, the object would sink. If an object were less dense than the water, the object would float. A simple raft made of wood can float because the wood it's made of is less dense than water. We say that wood is positively buoyant.