The term range in poker is used to describe all of the hands a player could realistically hold in any given situation. Poker players use all the information available to them to put their opponents on certain ranges, including betting patterns, their position at the table, general tendencies, and more.
Poker range is not a static thing. It changes as the hand progresses. An experienced player will, for example, put an opponent on a certain range before the flop, and this will be their starting point.
Once the flop comes, based on the previous assumptions and the opponent’s actions, they’ll adjust their range perception, adding or removing certain hands. In most cases, a hand range gets narrower with every betting street as more information becomes available.
Putting opponents on ranges is much more efficient and productive than trying to guess their exact hand. While it is usually impossible to know the exact two cards another player is holding, you can make pretty good guesses as to the scope of hands they’ll likely have based on the information at hand, i.e., their range.