Blinds
Forced bets (small + big) that start the pot before cards are dealt.
Take a $1/$2 nine-handed game. The player left of the button posts the $1 small blind, the next posts the $2 big blind, and both go in before anyone looks at a card. It folds around — UTG, the cutoff, the button — and the small blind is left holding 9-4 offsuit with a dollar already in. One more dollar buys a flop against the big blind, so a hand you'd muck from any other seat suddenly looks playable.
→ The pot starts at $3 before anyone has looked at a card.
Blinds are what make poker a game instead of a staring contest. With nothing in the middle, folding every hand would cost you nothing and no one would ever bet. They also create the quirk that drives half of preflop strategy: the blinds act last before the flop and first after it. A big blind who calls a raise plays the rest of the hand out of position, checking or betting into players who already saw what he did. Good players push on this, raising wider from the button and cutoff because the blinds have to defend with weak hands and bad position.
The common leak is treating posted blind money like it's still yours. It isn't — once it's in, it belongs to the pot. Calling a raise to "protect" your blind is the same sunk-cost thinking that turns a small forced bet into a big voluntary loss.