Note: Although this is about using stats in Poker Tracker, it should work just as well for similar apps like Holdem Manager or Poker Office.
As always with statistics, it is important to remember that it's useless before you have a decent amount of information in the database. You will have to decide for yourself when the stats are useful. In general you might start to see some trends after about 100 hands, but even then it's not very useful. Calming
The statistics displayed in the HUD by default in Poker Tracker (and a good place to start) are just three in addition to the number of hands recorded. You can also have your own stats on the table if you want to, but it's better to use "session stats" in that case instead of showing your combined stats, because it eventually becomes very large and effects performance.
VP (or VP$IP) is short for Voluntarily Put Money in the Pot and is a basic measurement of how loose or tight a player is. The formula is: (total times voluntarily put money in the pot / total hands played) *100. This includes every time a player puts money in the pot voluntarily - bets, raises, calls, etc. - except for posting blinds. What numbers are loose or tight depends on the game and the number of players. In a Texas Holdem 6-Max game, for example, a number below 25 can be considered reasonably tight.
A very simple stat, but useful as a measure of your opponents' aggression, it means Pre-Flop Raise % - how often someone raises before the flop. In other words, players with a high PR and low VP are likely tight-aggressive, high PR and high VP equals loose-aggressive and so on, but there's more to it than that;
Aggression Frequency measures a player's aggression after the flop. The formula looks like this: [(Total Times Bet + Total Times Raised) / (Total Times Bet + Total Times Raised + Total Times Called + Total Times Folded)] *100. What is a "good" Aggression Frequency? When it's above 40 you could say it is aggressive - most good (aggressive) players have an AFq somewhere between 40 and 60.
See also: Poker Tracker 3 Review