The Archimedes principle states that:
The amount of force pushing a body up in a fluid equals to the weight of the fluid that such body displaces.
This seems complicated, but it's actually simple:
Suppose you have a ball of volume equal to 1L. Say that ball weights 0,5kg. However, 1L of water weights 1kg. So the ball floats on water.
Another way of saying it:
If an object has lower density then water, it floats.
If an object is lighter then an equivalent (same volume) mass of water, it floats.