After the flop, your hand is more or less defined. Before the failure, which has seen only 2 cards total 7 cards that will eventually be at stake. The flop gives you 3 more cards, so now you are seeing 5 of the 7 cards in total that eventually will make your hand. As a result, the strength of your hand will change far more on the flop of what will be on the turn or river, with the exception of the time you draw in color or stairs. For example, before the flop, an Ace and King suited, as AhKh is a good hand, with the potential to become a huge hand.
After all, with a lot of luck it could become a royal flush, the best hand possible. A little more likely is something like top pair, with the second best card available, a pair of Aces, with the king as his second card (kicker), being a very good hand. However, if the three cards in the flop are 2, 6 and 9, without heart, you have nothing, but a weak tie. The best you can hope for is that everything is checked, and an ace or a king comes on the turn. Its potential has resorted to garbage, and if there is a bet that must fold unless the pot is very large. At the other end of the scale, you can start with something like a J and Q inadequate, which is expected to be playing in only one of the two last positions or blinds, which is a weak starting hand.
