There are several parts to this answer.
Energy for human life comes from oxidation/reduction chemistry. Potential energy is available in the difference in oxidation state between two reacting chemicals. (As they exchange electrons.) In our case, we react oxygen gas (the oxidized chemical) with organic carbon (the reduced chemical.)
However, our bodies are also made of organic carbon - relatively reduced compared to our environment. Any strong oxidizer can attack the molecules that we're made of and damage them. So we need oxidizers to live, and yet they're also toxic.
Some polyphenolic compounds seem to act as non-specific electron carriers - they are very easy to oxidize and reduce, without being destroyed. They basically absorb oxidizing power from their environment. When that happens in the human body, they are thought to "protect" the surrounding tissues from oxidative damage. They sort of act like the zinc anode on a boat motor, that prevents corrosion in sea water. These are what are commonly referred to as "antioxidants," although technically, our entire bodies are reduced, and therefore "antioxidants." They may be protective in some cases, but too much of them in the wrong place would short-circuit our metabolism.
However, we now know that many soil bacteria are able to get their primary oxidizing power from these polyphenolic compounds. They "breathe" them, in other words. In that case, the polyphenolics act kind of like antennae that absorb energy from their environment. We don't know of any mechanism where humans can get energy this way, but some of our intestinal bacteria probably can.