Equity
The share of the pot a hand will win on average if all remaining cards are dealt.
Equity is your hand's chance of winning, expressed as a percentage. A flush draw on the flop has roughly 35% equity vs a single made hand. A pair of aces vs a pair of kings preflop has about 81% equity.
Equity changes with every street. Cards that improve you (outs) push equity up; cards that improve your opponent push it down. "Realising your equity" means you actually get to see all five cards — opponents pricing you out reduces your realised equity, even when raw equity is high.
Use a free equity calculator to build intuition: practise estimating equity for common spots, then check yourself.