Outs
Unseen cards that improve you to the likely best hand.
You're on the button with K♦Q♦ and the flop comes A♦J♦4♣. Nine diamonds are still live for the flush, plus three tens for a Broadway straight — the fourth ten is a diamond you already counted — so twelve outs, about 45% to hit by the river against one player. No made hand yet, but plenty going for it: enough to call, sometimes enough to raise, depending on the price.
Counting outs turns a hunch into a number. A flush draw is nine outs, near 35% by the river. An open-ended straight draw is eight outs, about 31%. A gutshot is four outs, around 17%. Set that against the price you're paying: call $20 into a $100 pot and you need roughly 17% to break even. Now the call is arithmetic, not a guess.
The trap is counting every card that improves you as an out. Top pair with a weak kicker can catch one of two cards for trips — but on a coordinated board those same cards can fill a straight or flush for someone else. An out only counts if it gets you to the best hand, not just a better one than you held. Miscount that and you talk yourself into calls that bleed money over the long run.