Definition of hand range
The "hand range" is the range of hands an opponent can have. Depending on our position, our hand range will vary. Obviously, a player won't play all the hands he receives. He'll make a selection. Several factors will help him make his selection. His starting position is one of them. If you're the first player to speak (the UTG position), you'll play a smaller range of hands. If everyone else has folded to us on the button, we'll have a wider range. If a player has already made a 3bet in the hand, we'll have an even smaller range of hands to play.
Why evaluate the hand range?
One of the most important skills a poker player can have is the ability to assess his opponent's hand range. Why is that? Because it tells him which hands he can play. For example, if we estimate that in a given situation a player will have only 5% of the hands in his hand range, we know he'll be very strong. And to face this range, we'll want to have a very strong small range too, otherwise, in the long term, we'll lose money. It's also important to make sure that our hand range isn't too easy for our opponents to guess, and to do that, we'll need to balance it well.