A pop filter, or popper stopper, is a round disc with a sheet of material stretched across it (usually fabric, but sometimes metal). It attaches to a microphone stand and is positioned between a singer and a microphone to reduce the effect of plosives and sibilance in studio recording.
A plosive is a fast rush of air resulting from certain vocal sounds. When this hits the microphone it causes the diaphragm to distort, resulting in a pop. This plagues recording and mixing engineers the world over. Plosives typically occur on B's and P's, and F's but can also occur on R's, H's and CH's.
Almost as bad as the plosive is sibilance. Sibilance is defined as a high pitched hissing noise that occurs from vocal sounds such as S's and T's. Put simply, it is the result of high frequency sound waves pushing air across a mic diaphragm. It's the sort of thing you won't notice until some one points it out, but when they do you won't stop hearing it. It can be controlled to an extent in post production, but doing so can compromise vocal tone quality.