The upward force that makes things float is called buoyancy. Buoyancy is the result of pressure differences in a fluid (liquid or gas) that act on an object submerged in that fluid. According to Archimedes' principle, an object will experience an upward buoyant force equal to the weight of the fluid it displaces.
When the buoyant force is greater than or equal to the weight of the object, it will float. If the weight of the object is greater than the buoyant force, the object will sink. Essentially, buoyancy is what allows objects, such as boats and balloons, to float in water and air, respectively.