The tendancy of a body to float or sink in water or any other fluid. Most people will naturally float in water, especially if they fill their lungs with air.
Gravity pulls a person's body downward by a force equal to their weight.
The difference between these forces is a person's net buoyancy. A PFD is very light weight, but displaces enough water to make the PFD and the person wearing it very buoyant. It also follows that the people hardest to float are those with compact, dense bodies. These tend to be people with athletic body builds, with a lot of bone and muscle mass, and not much fat. Fat is not as dense as muscle and bone, so people who are overweight can actually be easier to float than someone who is much smaller and leaner.