The term range in poker refers to all possible hands a player can realistically have in a given situation.
Poker players use the concept of the hand range to better gauge what their opponents might be holding and adjust their strategies accordingly.
Putting a player on a certain range requires combining several factors, including their general tendencies, their position, all the betting action, and any particular reads.
Using ranges is much more efficient than trying to put a player on particular hole cards and, when done correctly, can be extremely helpful in the decision-making process. Learn more about ranges in poker.