What Does Negative Feedback Mean?

If i’m correct it means that signal get fed back into the filter but flipped around the DC axis, correct? In some filters I can hear the difference of for example +50% and -50% feedback and for some filters it seems to be exactly the same.

You are correct.
Negative feedback is sometimes helpful for avoiding clippings in the sound but sometimes give the wrong feel (more metallic, like speaking into an empty tincan) of the to the sound. If you are afraid of getting DC ofsets use negative feedback instead of positive. Try playing with the Comb filter, it has the most audible difference between positive and negative feedback since it doesn’t process the back-fed signal.