The top pair poker term is popular in flop games and it refers to the situation when you match one of your two hole cards with the highest-ranking card on the community board.
To have a top pair, it doesn’t need to be the strongest card in your hand, as long as it makes the strongest pair on the board.
For example, you have a K, 9 in your hand. The flop produces a 9, 5, 2 community board.
As 9 is the highest-valued card on the board, you effectively made a top pair. Oppositely to this, if you had a K, 2, and the same cards came up, you would have made a bottom pair.