Suited connectors
Two adjacent cards of the same suit (like 8♥7♥). Small now, but they make the straights and flushes that stack people.
Say you hold 8h7h in the cutoff and the flop comes 9h6h2c. That's an open-ended straight draw and a flush draw at once: any 5, any 10, or any heart gets there, 15 outs in all. A combo draw that size is a slight favorite over top pair with two cards to come, and it stacks the player who flopped a set. That's the point of these hands. The two cards work together, so junk nobody respects preflop can be the best hand at the table by the river.
Cheap to enter, expensive to misplay. Most flops give 8h7h nothing — no pair, no draw — and that hand should fold and cost you nothing more. The money comes from the times you connect big and get paid, which is why position and pot size matter. Calling a raise from under the gun is usually a mistake. From the button or cutoff, especially multiway where the implied odds are there, it's a fine speculative call. These hands want company, not a heads-up pot against one aggressive raiser.
The beginner mistake is playing them backwards: calling because the hand "looks suited," then folding the instant the flop bricks, never around for the implied odds that justified the call. The other version chases a gutshot and a backdoor flush like every suited-looking flop deserves another bet. Suited connectors pay off when the big draws or made hands arrive. Everything else is a quick, cheap fold.