When you pair the highest card on the flop with one of your hole cards, you have top pair.
When determining the winning hand after the river, in the case of two or more players having equal top pair hands, the highest card outside the pair (referred to as the kicker) is used to break ties.
For example, if you are holding Ace King and the flop came as King, Nine and Deuce, you have flopped top pair with top kicker.