24=Voluntarily Put $ In Pot (VPIP), means how often you raised or called pre-flop, doesn't include posting blinds (hence the voluntary bit).
20=Pre-Flop Raise (PFR), how often you raise pre-flop. This number will always be the same or lower than VPIP, it is used to see how aggressively a player plays pre-flop. I think a standard(ish) rule is that it should never be bellow 50% of your VPIP (24/11=too passive pre-flop, 20/18=standard TAG).
VPIP/PFR are very useful to tell at a glance if someone is a fish (50/3) or someone is a TAG (19/17), sometimes you also see LAG stats (24/20 in FR, 32/28 6max) and then it's anyone's guess if they're a fish or decent. Those numbers are all just estimates though.
Also it's important to remember that players with the same stats don't play the same, their stats are a very small part of a bigger picture. However when posting a hand for analysis it helps us to see if he's a megafish or playing a TAG game, keep in mind though that stats are only as important as their sample size. Anything less than 50 hands and you need some extreme stats to determine anything (i.e. his VPIP is 70 after 50 hands, he's always going to be a fish, but if it's 25 then you don't know if it's going to end up as 30 or 18), anything more than 100 hands and the VPIP/PFR will have converged fairly accurately and you will only see changes of a point or two. There's also other obvious stuff like a table becoming short handed for a while to throw off their stats, so anything like this you need to keep an eye on.
Finally, these stats only apply to pre-flop. How they play pre-flop and post-flop can be entirely different.