Whatever type of poker you are playing, free poker games, pay poker or poker with friends for fun, watching the cards which make the flop is so important, especially when you are new to the game.
As the shared cards on the table are played by all of the players in a game, there is always a chance that another player is holding the same cards as you do, and hold a higher kicker. Take this situation, for instance: Two pair is a very common hold in hold'em. Let's suppose that player A has two sixes in the hole. That's a turning hand, as he now has a pretty good hope of getting another to get three of a kind, and he's going to get three cards in the flop instead of one at a time to see if he can get that other six.
The turn comes an eight, eight, and deuce. Now Player A has sixes and eights. Player A is now looking good to win the game over Player B, who is holding an ace, king as there are still 2 more cards on their way and this player will need to catch one of these in order to beat Player A.
Along comes a ten. Then along comes a ten. Now Player B has taken over and Player A cant beat him. Player B has tens and eights with an ace kicker. Since only five of the seven cards are played to make a hand, Player A has eights and tens with a six kicker. Actually, he's ended up with three pair which aren't worth a damn. If the final community card hadn't been a ten, then Player A would have won, but Player B has now won the pot because of his 'ace kicker'.
If you are new to hold'em, have a go at playing free poker games and see how you get on before betting any real money on the cash tables.