Something floats if you put it on the top of a liquid like water and it stays on the top.
Things float because they are less dense than the liquid they are in. For example, the duck in the picture floats because the amount of duck that is below the water level weighs less than the amount of water that would be where the duck is - a concept called displacement.
If you are in a swimming pool with a foam kickboard that floats, try holding it under the water. You can also try this with a ping-pong ball or a polystyrene foam ball in a big bowl of water. It is very hard to get something that floats to go under the water, and if you hold it right down and then let it go it might even pop right out of the water! It is this force that holds floating things up.
The opposite of float is sink.