The nuts
The best possible hand on a given board. Nothing beats it, only ties it.
Board of 9h-6h-2h on the flop, two blanks by the river, and you hold Ah-Th on the button. Three hearts sit on the board, your ace and ten fill out the flush: ace-high, the best hand these hearts can make. The board isn't paired, so no full house or quads. The hearts don't run in sequence, so no straight flush. Nobody can beat it. That's the nuts.
Holding the nuts changes how you play. Bet and raise freely — whatever anyone turns over, you don't lose the pot. That's why you chase nut hands instead of hands that are only probably ahead. A set, two pair, top pair top kicker can all run into something bigger; the nuts can only be tied. And it does tie sometimes. If the board itself is the best hand — say a rainbow A-K-Q-J-T, Broadway showing on the table — everyone left is playing the board and the pot splits. The hand held up. It just didn't win alone.
Beginners confuse "very strong" with "the nuts." A king-high flush feels fine until someone flips the ace of that suit and drags the pot. Before you commit your stack, count what beats you on this exact board — not what usually beats a hand like yours.