Answer:
Microphone feedback is caused by the feeding the same signal back to the microphone, usually through a monitor speaker in a live concert situation. Because most microphones are not linear in frequency response, the feedback is usually being exaggerated and is heard as either a slow low sound or a really fast and spiky tone. For example a Shure SM58 has a spike around 1khz that usually feeds back really easily if you put your hand on top of the grille. If the feedback is really slow, it is usually below 1khz, if it is fast, it is usually above 1khz. You can try to remove feedback with better gain staging, better microphone choices and with an equalizer.