Feedback

the typical situation is a microphone gets too much signal of the speakers, it also listens to the speaker, so that signal will be amplified and accumulates, especially the resonance frequency will be emphasized.

when the output signal of a delay goes back into the input and gets delayed again and again and again, most of these feedbacks are done after damping the signal with a filter, in most cases a LPF (Lowpass).

the stronger the feedback, the more it is one specific frequency the feedback is on, so sometimes it helps to filter it out, thats what a so called feedback destroyer does, it's one or a small set of narrow Bandpasses (filter).