"Feed on" means to eat or consume. So to feed on something means to get sustinence from it. "Everything was feeding on everything else" sounds odd out of context, but in context it might mean something like "Everything was eating each other" or "Everything was interdependent (for some reason)"

Longer context would help, but perhaps it is referring to feedback: every output becomes an input to something else, making the situation more intense. "She thought she could handle the consequences of lying, but now things were getting out of hand. Every lie she told required more lies to explain it. Everything was feeding on everything else, and she knew she couldn't sustain the situation for long." "The party was incredible. The wonderful music inspired the dancers, and the magnificent dancing inspired the musicians. Everything was feeding on everything else, and together the music and the dancing rose to a perfection that none of the people at the party had ever seen before."

I saw it when I was looking the phrase "feed on" up. It's an example on Longman dictionary. Here's the link: https://www.ldoceonline.com/dictionary/feed-on

"feed on" means to eat or consume. This sentence is a little strange because everything eating everything sounds crazy... But maybe that is the point. Where did you hear this sentence?