Definition of pf
Pf stands for "preflop," which is any action that occurs before the flop. Not every poker hand sees a flop. Not every poker hand sees a turn or a river either, but every poker hand has preflop action. Preflop is the action that occurs before the community cards are dealt (flop + turn + river). When playing preflop, only our two cards in hand count since there are no community cards dealt yet. When all players still in the hand have put in the same amount preflop, we can then reveal the 3 community cards on the flop.
Preflop, the best controlled street
Generally, the theory of preflop poker is fairly well understood among poker players. This is the place where poker players make the fewest mistakes. The further into the hand you go, the more complicated things get. Even at the small limits, the most novice players will still have a good idea of the optimal way to play their cards before the flop. If we can give you any advice, try to be the pre-flop raiser when you get into the action of a pot (in short, try to take the initiative, you'll have more success post-flop).