Definition of range
A poker player's range is his range of possible hands. By range, we mean "all possible hands according to the player, his position and the previous action". For example, preflop, all players have a 100% range, meaning they were dealt 2 random cards and can have any of the 1326 combinations. Now, let's say a player raises preflop; here, his range is no longer 100%. If he is in early position, his range is even smaller. So he goes from a 100% range to say a 15% range. A range is a range of possible hands at a given time.
? What is the use of knowing the range of a player?
? make good calls and good folds! When you play a hand, you think about your cards, how they connect with the community cards, but also about your opponents' range to see how your hand fares against their range. For example, a player calls in UTG position. You call in SB position with 88. The flop comes 2-5-7. Your opponent bets and you call. The turn is a J. Your opponent bets or you call. The river is an A and he pushes all in. Every time he bets, his range shrinks. Here, your opponent might have a range of AA, AK, AJ, JJ, 77, AQ. His bluff range is very limited. Against such a range, your 88 is a fold on the river. Against a huge range, it could be a call.