The VPIP measurement is a commonly used indicator in all poker calculators. The reason for its inherent importance in online calculating tools is that it clearly shows one of the main attributes of a poker player - and that is his willingness to get involved in pots.
Your opponent's willingness to get involved in the play is translated as "Voluntarily Putting money In to the Pot" or VPIP as most online player's know it. VPIP is an extension of the classical profiling scale of poker player's identities where passive aggressiveness and the tight loose scale interact to label your opponent. It's the quickest way to determine your opponent's style of play and when combined with other indicators the VPIP will generally be an accurate guideline for game planning against a particular opponent.
Whenever a player goes into a pot either by calling or raising that adds to his VPIP and is expressed as a percentage of the total hands he has seen. So let's say that after the 20th hand you have raised once, called a raise twice and filled the small blind once. That means you have voluntarily entered the pot by taking chips from your stack and putting them into the pot four times. That means you have a VPIP percentage of (4/20) 20%. That amount is well within a normal playing range of straight in poker, but really 20 hands is a short range to measure.
A poker calculator like Tournament Indicator will keep tabs on yours and your opponent's collective statistics for up to two years covering thousands and thousands of hands. Once you have some data on your opponent though, you will need to know what the VPIP percentage actually indicates as to the player's strategy.
A VPIP percentage between 24% and 30% is generally thought to be straight up, position and hand card strength. You can normally expect a competitor in this range to be either be playing hole cards with inherent strength, position blind steals and position potential hands like connectors.
If your opponent has a VIP higher than 30% you can assume all the hands above but added to that are connectors, suited cards, any ace, any two big cards and any pair from any position. If you see someone playing even more like 40% VPIP, well it's hard to classify junk but that's what they may be holding.
A VPIP percentage lower than 24% is usually on the tighter side of things and will not be raising to steal blinds, will not be calling with connectors that have good odds to do so, and will only be in a pot ahead of you if he has raised.
For practice, next time watch around your table and measure the VPIP of all players, including yourself. There is really no right or wrong percentage as game conditions may demand a certain strategy, however it's hard to win long term with a high VPIP.