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Buoyancy is an upward force on an object, exerted by a gas or liquid, that pushes against the object’s weight.  If an object can float, this means that the buoyancy force is more than the object’s weight.  If the object cannot float, then the buoyancy force is less than the object’s weight.  Buoyancy and density are what allow boats and hot-air balloons to float.  Density is the mass of an object divided by its volume.  In other words, it is the amount of “stuff” in a set amount of space.  A boat can float on water because there is air inside the boat that makes the boat less dense than the water.  A hot-air balloon can float because hot air is less dense than the colder air around the balloon.