A Pop Filter is used with a microphone to eliminate particular sounds. When saying words with hard consonants, the mouth can cause a sudden air rush, similar to a popping sound. Saying the word 'Pop' out loud will demonstrate the effect. The first 'P' in 'Pop' can overload a microphone for a short duration. A Pop Filter prevents this effect. A Pop Filter can be constructed from many materials and set up in different configurations, but most studios use a round nylon or plastic frame with a mesh placed in front of the mike, between the mike and the singer.
X
______ X
+------------- | X Singer
| | |/_____ X /_________
| | |\ X \ Sound
+-------------______| X
X
Microphone X
Pop Filter
Normal sound passes through the mesh, and the sudden popping sounds are dispersed. Some folks mistakenly think the Pop Filter is a spit guard, but this is a secondary role for the device. Pop Filters should be cleaned regularly. | 
