Feedback control is a problem which arises when you have a microphone and speakers which can feedback, or when the sound from the speakers gets directly into the mic and then the system re-amplifies the feedback and a very vicious circle is made.
The easiest and best way to avoid feedback is to keep the speaker who holds the microphone away from the loudspeakers. Unidirectional mics, combined with proper speaking technique, can all but eliminate the problem. However, getting back to reality we are faced with quiet speakers (those who are speaking) combined with improper usage or placement of the microphone. Watch professional performers when they use their hand held mics; they will virtually be eating their mics as they will have it very close and correctly placed to their mouths.