Poker is a game of aggression. It is virtually impossible to be profitable over a long period of time without having an aggressive play style. At the same time, playing aggressively can open up exploitable leaks in a player's game if they are not knowledgeable and cautious in how they execute their aggression. It is important for a successful poker player to be aware of both their own level of aggression and the aggression of their opponents. When using a database or a heads-up display in online poker, the best statistic to use for this is PFR.
PFR stands for “pre flop raising”. It is calculated by taking the percentage of hands a player raises before the flop and dividing it by the total number of hands dealt. For example, if a player raised 15 hands before the flop and was dealt 100 hands, his PFR would be 15. It is important to note that the number of hands a player plays (raised and unraised) has nothing to do with calculating PFR, so playing 20 hands and raising 15 of them would not give a PFR of 75.
PFR poker is part of every level of poker strategy, so understanding what different PFR numbers mean gives a player an edge against every type of opponent. Someone with a low PFR is generally only raising with premium hands. This information can be used either to simply avoid the player with weaker holdings or (for more advanced players) to narrow down the tight player's hand range, which makes decisions after the flop easier and more accurate. A player with a high PFR is going to be raising a much wider range of hands, and can therefore be exploited either with reraises before the flop or by outplaying them after the flop.
PFR can also be used in conjunction with VPIP to get a baseline sense of a players skill level. Players with a very large gap between VPIP and PFR (like 40 and 10) are typically going to be weaker players, while a player with a small gap (like 22 and 20) is generally going to be a more competent player.
Since this is quite an important parameter, you can find various tools to help you use it to your advantage. The Magic Holdem Poker Coach is a great example of such tool. It shows your PFR poker rate in the relevant point of your game. You can then adjust your game according to the recommendation of the poker coach.