It stands for No-Limit Texas Hold'em. It's one of the most popular and widely played variants of poker. In NLHE, players receive two private cards (hole cards) and combine them with the five community cards (i.e. flop, turn, and river) to make the best five-card hand.
What sets NLHE apart is that there is no limit on the amount a player can bet or raise during each betting round. Players can wager any amount, even going all-in, risking all their chips at any point in the hand.