Definition of postflop
Postflop means "after the flop". Everything that is played after the three community cards have been revealed. It is often said that this is the most difficult stage in poker. Some poker players will be "shortstack pros", meaning that they will have 20 big blinds at the table at all times. Thus, they will limit postflop decisions. Most of the time they will be all in preflop, so they will not have to make decisions postflop. The preflop strategy is generally well understood, even by novice players, but the postflop action is more complex.
Some tips to help you in 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, the position will still have an advantage over you, without you even realising it. Next, try to get the initiative. A player has the initiative when he has made the most aggressive move on the previous street. The player with the initiative will be able to bet more often on the flop and his bluffs will be more successful because he has shown strength earlier in the hand. In closing, take notes on each of your opponents. You don't play the same way against all players!