Usually it's VP$IP/PFR%/AF
VP$IP = % of the time person put money into the pot voluntarily (meaning raises or calls preflop, does not include checking from the BB) preflop.
This number can be used to see what kind of
hands this person will go to the flop with. If a person has a VP$IP of 50% (VERY HIGH) it means that they are seeing flops with any hand better than average.
PFR% is very straightforward, it's the % of the time they are raising preflop. This is significant because it shows how often they'll raise preflop. If this is 50% it means they are raising with any hand preflop and not to take their raises PF very seriously. If it's 0 it means they will still limp with premium hands (or sample size is too low, have to watch for that)
AF = Aggression factor = (bet + raise)/call
So if their AF is 5, it means they bet or raise 5 times as often as they call. It's hard to understand without just seeing numbers over time and realizing what is high and what is low. This is also distorted by folds, as if someone simply either raises or folds, but folds 90% of the time, it will show them with an infinite aggression factor.