Audio feedback is a form of positive (but undesired) feedback. It describes the annoying noise that can be heard when the output of the loudspeakers is looped back into the microphone, creating an amplification effect.
The immediate solution is to reduce the volume to the speakers. That will end the unpleasant howling. A better solution is to lower the input level of the microphone which is producing the feedback.
A narrow band filter applied at the feedback frequency is the best solution of all, although it is difficult for amateurs to do this unless they have proper equipment and training.
Electric guitarists such as Jimi Hendrix will often deliberately generate feedback with their amplifiers and use it for musical effect.