Audio feedback refers to when sound loops through a closed system, reenforcing and producing unintended noise.
This usually occurs when a microphone picks up sound from a sound system which “feeds back” through the microphone, into the speakers, and so on in a loop.
This can be avoided through a combination of careful audio engineering (reducing the volume and frequencies which feed back within a given space and sound setup) and proper microphone handling to avoid placing microphones too close to a coupled sound output source.