Slop.

So your body naturally creates certain oxygen containing molecules called free radicals. These are created in response to stress, immune response, and environmental stressors like UV light, radiation, and smoke. Free radicals are highly reactive as they lack a valence electron. Due to this reactivity they can damage molecules that make up your cells, including DNA, and lead to long term health issues. This is called oxidative stress. Antioxidants are molecules which have an extra electron and readily donate it to molecules that are lacking one, in this case the free radicals in your body. Their importance is in this function of reducing oxidative stress.

I don't remember why, but certain compounds break down into "free radicals", or single oxygen atoms (as opposed to double-bonded oxygen molecules) that very easily bond to other compounds in your body and break them down.

Antioxidants cut down on the free radicals in your blood (typically by bonding with them first, I believe) to prevent them from destroying important compounds in your body.