Definition of VPIP
VPIP stands for "Voluntary Put Money Into Pot". It is the indicator of the percentage of time you put money into a pot of your own free will. This is the most important and primary statistic in your HUD. If you have a tracker like PokerTracker and you display a HUD (heads-up display) on your table, the first two basic stats you will see are VPIP and PFR (preflop raise).
Why is VPIP important?
Knowing a player's VPIP is important because it gives us an idea of their playing style. If we are playing a shorthanded table, for example, we know that at most every third hand is playable, which should give a VPIP of about 33%. But if you see a player with a VPIP of 75%, he is playing too many hands and therefore will either have to bluff a lot to make those hands profitable or he will be a very poor player playing all those hands. ? Conversely, if a player has a VPIP of 8%, you know that he will only play very good hands and you know that you will need to have very good hands to play against him.