The percentage of hands a player voluntarily puts money in the pot.
An average 6-max player is 20-25%. Read it next to PFR (preflop raise rate). VPIP 40% or more with PFR at half of VPIP or less is a player who doesn't fold (station); VPIP 15% or less is tight (nit).
At the table
If a player put money in voluntarily (called or raised) in 25 of 100 hands, their VPIP is 25%. Hands where they only posted a blind and then checked or folded do not count.