The full set of hands an opponent could hold given everything they have done so far — not the single hand you imagine them having. Thinking in ranges is the shift that separates guessing from reasoning, because you are almost never able to name someone's exact cards, and you almost always know something about the group they belong to.
A range starts wide and narrows with every action. An early-position open eliminates most weak holdings. A three-bet cuts it further. A continuation bet on a low, connected board keeps overpairs and draws while a check often keeps the hands that missed. By the river a range that began as two hundred combinations may be down to a handful, and those combinations are what your decision is actually against.
The useful move is to count rather than to picture. If a river raise can be four value combinations and two bluff combinations, the pot is offering you a price and you can compare the two directly. If you instead ask whether he has it, you will answer with whatever you fear most.
Two habits make ranges practical. Narrow honestly — it is tempting to strip out every hand that beats you, and the result is a range built to justify a call you had already decided to make. And keep your own range in view: the reason a bluff works is that your betting range plausibly contains the hand you are representing, and if it does not, no amount of bet sizing will fix it.