A feedback buster is a rubber plug designed to fit loosely into the sound hole of an acoustic guitar to help cut down the feedback loops that tend to occur when playing an amplified acoustic guitar at any sort of volume.
The effectiveness of these things varies greatly depending on the design of the guitar in question, and how much the guitar is being amplified. A good sound tech should be able to cut back the frequencies causing the feedback anyway, so they can be kind of redundant. They also tend to make a guitar sound a little bit more flat, as though you're palm muting a TINY bit.
The other downside to the very simple technology that is the feedback buster is that it will not function on any guitar that doesn't have a regular round sound-hole, so if you play an old Maccaferri with the D shaped soundhole, you're buggered. If you play an Ovation with a series of smaller holes on the upper bout, you're buggered. If you play an old jazz box with no sound hole at all, you're buggered.
Get a good sound tech, or just turn down.