When someone mentions “feedback”, you might imagine the squeal of placing a microphone too close to a speaker. However, adding small amounts of feedback or “resonance” in a filter module is a popular trick to change its character, used to create many instantly recognizable sounds. (And yes, you can make some filters squeal as well, if you like.)
Filter resonance is also introduced on page 18 of Patch & Tweak which you can get here.