VPIP/PFR are preflop stats and an indicator of preflop aggression.
AF is a postflop stat and an indicator of postflop aggression. It's a measure of aggressive vs passive actions. In this case, AF is calculated as (Bets + Raises) / Calls. Other similar calculations take into account checks and folds. So the more aggressive someone is postflop (the more they bet or raise, as opposed to calling), the higher their AF will be. An AF of 1 is a very passive player. An AF of 5 is a fairly aggressive player. Of course this also depends on sample size.