Polyphenols are antioxidants! Antioxidants are very good for you, and also help slow down the oxidation process in olive oil.
Phenols die off over time, and are a really good indicator of freshness. Also, a high level of polyphenols means your olive oil will last longer!
Currently, there is no rule about polyphenols or levels required to meet the extra virgin standard! In order to qualify for ultra premium though, an olive oil has to have a minimum phenol level of 130.