What is the purpose of a phytonutrient? Are they mainly antioxidants? I read olive fruit's hydroxytyrosol has the highest orac of antioxidants.

Beware of salesmen. People will try to sell anything, so watch out for products that claim to do beneficial things in lab studies, animal studies, very small studies, or studies that are not done in people like you. A phytonutrient is a plant-derived compound (chemical, protein, material) found to have positive health effects... But most are not studied, purified, nor quantified like real medicines have to be.