A poker range is the range of poker hand combinations that any person might have at a given time. You use poker ranges during playing poker in order to value not only your hand, but most importantly your opponent's hand.
A poker range is important because in poker you are basing your betting off of evaluating your hand compared to the other player's hands. Therefore you use the poker range to help you determine when to call a bet or when to fold your hand if you think that you don't have the best one. However, while poker ranges are great to help you get some idea of the odds of what hand your opponent has, you can't entirely play based on poker ranges.
While a poker range gives you a good educated guess on the strength of your opponent's hand, you still have to take into account the tendencies of the other players at the table. Since while you could have a good range of your opponent's hand based on how they bet on the hand, you still need to take into account whether they play tight (conservative) or not. What this means is that if you have a range of cards that is quite large, and the person bets on the pre-flop when he hasn't bet pre-flop all game that is a sign that you should adjust your range to be higher.