Buoyancy is a neat force, which is that upward push a fluid places on an object. Swimming would be a bit difficult if buoyancy was not there, since then everything and everyone would just sink!
Archimedes figured out long ago that the buoyant force = the weight of the displaced fluid. This video gives a brief example and proof to something that you may be aware of, that whether things sink or float depends on the relative densities. In chemistry, oil floats on water if it is less dense than water - well, here is the 'proof' of this phenomenon using Newton's 2nd law. I hope it helps!