VPIP - tells
you how loose some one is preflop, 10 is nitty, < 30 is very loose, between is about average. That is my experience and it does change by levels.
PFR - How often are they raising rather than calling. I don't use the number so much as look at how it compared to vpip. example 10/10 means nit and if they are in they are raising preflop. great candidate to set mine against. on the other hand I see 65/8 I know they limp all kinds of garbage but tighten up when they are raise preflop (total fish numbers by the way) A 30/27 gets a little ugly to play against Lag but they aren't usually any good.
AF - just means tells you how likely they are to bet. for example a 10/10/4 is a nit that will always bet. so in that case if you set mine them, and hit your set you could donk the flop, they will raise shove over the top and you stack them. easy peasy. on the other hand a 65/5/.7 is a total donk calling station flush chaser and you just value bet the crap out of them and fold if the draw gets there and they bet it. or check call a reasonable bet.
This is just my take on this stuff. I don't claim to be an expert but I struggled w/HUD stats initially is this is what I have learned so far.
also you use the steal % to balance the PFR. What I mean is the higher thier Steal % is the tighter they are in early positions. So a 25/20/3 seems like a wide PFR range but if thier Steal % is over 30 or something then when they raise in EP thier range is much tighter than it would initially appear, maybe only including AA, or KK.