To plop is to drop something (or yourself) with a short sound. The sound itself is also a plop — like something landing in water without much of a splash.
The sound of a plop is abrupt and hollow — you could also call it a plunk or a or a plonk. You might plop an ice cube in your glass of water, or watch a flock of bird plop themselves on the surface of a pond. Plop is imitative or onomatopoeic (it sounds like its meaning), and it first appeared in the 1820s after the brief popularity of the alternative word plap.