I'd like to know how antioxidants affects the human metabolism and which ones are essential for metabolism processes.
This mainly depends on which kind of antioxidants you're talking about.
Antioxidants keep the oxidative balance of the cell and quench free radicals which are generated by the mitochondrial respiratory chain. Very simplified: Antioxidants are thought to be anti-ageing compounds because free radicals destroy cellular compounds, which can be related to the ageing process.
I would be very careful with products claiming to contain antioxidants and be therefore anti-aging. This whole anti-ageing marketing is far from being scientifically proven.