If you have two cards and one of them makes a pair but the other doesn't, the second one is your kicker. So if you have AQ and there's Q74 on the Flop, you have "top pair top kicker" because the Q is the highest possible pair and the kicker, A, the highest possible card. And KQ would be top pair second-best kicker and so on. If two people have the same pair, the one with the better kicker usually wins.