Top and Bottom

1. Pairing your hole cards with the highest- and lowest-ranked cards on the board. For example, on a board of 9-3-4-5-Q, holding Q-3 would mean having top and bottom pair.

More Examples

On a flop of 2-7-A, if you have A-2 then you have flopped top and bottom pair.

If you hold T-2 on a T-7-4 flop and the turn is a 2, you have turned top and bottom pair.

If you hold A-6 on a A-Q-T-7 board and the river is a 6, you have rivered top and bottom pair.

Expanded

Generally speaking, novice poker players tend to overvalue top and bottom pair. It obviously depends on the situation, but let's say for example you're on the button with As-5c and open for 3x the big blind. The small blind calls and you're in a heads-up pot. The flop comes down Ah-Qh-Td and your opponent check-calls. The turn is the 5h, which gives you top and bottom pair. When your opponent check-raises your turn bet all-in, you should definitely fold. Most beginning players will call simply because they have two pair, but if they were thinking about the hand even remotely, they would realize that the small blind's range crushes A-5. KJ and flush draws are a huge part of your opponent's range, as are A-T, A-J and A-Q. Sometimes your opponent might have a combo-draw, but even then you're not in great shape. For all intents and purpose, the 5 may as well be a brick, because since it's a heart, it actually strengthens the small blind's range and weakens your own, despite giving you two pair.