Definition of postflop
Postflop means "after the flop". Anything played after the 3 community cards have been revealed. It is often said that this is the most difficult step in poker. Some poker players will be "shortstacks pros's", meaning that they will always have 20 big blinds's at the table. Thus, they will limit the postflop decisions. Most of the time they will be all in preflop, which will prevent them from making decisions after the flop. Strategy preflop is generally well understood, even by beginners, but action postflop is more complex.
Some tips to help you with your postflop game
First, play in position as often as possible; you will already have an advantage. Even if you don't understand exactly why playing in position gives you an advantage, think that the position will still have an advantage over you, without you even realizing it. Next, try to take the initiative. A player has initiative when he or she has made the most aggressive action on the previous street. The player who has the initiative will be able to bet more often on the flop and his bluffs will be more successful because he will have shown strength previously in the hand. Finally, take notes on each of your opponents. Not all players are played the same way!