Hand ranges get narrower and narrower as the hand progresses. This is because the actions of our opponents tell us information about their hand. Let's say that if our opponent calls a bet on the flop, he likely connected in some way with it, a pair, two pair, a draw, etc. That way we can eliminate from his range all the hands that don't connect, over cards, missed connectors, under-pairs, etc.
For example: We raise in the CO and everybody folds except the button, who calls, and straight ahead we can eliminate a lot of very weak hands from his range like:
We can also eliminate hands that we think out opponent would re-raise pre-flop such as:
Say we go to the flop and the flop is:
We bet and are get called, now we can narrow his/her range even more to hands that connect with this board (hands that don't would fold to this bet), such as; A10 or 22.
We can now narrow our opponents range to:
1) A top pair hand.
2) A flush draw like A6
3) A straight draw like QK or 10Q
4) Two pairs.
5) Three of a kind.
The turn is dealt which is:
We now bet and get raised, now we can narrow his range to pretty much only strong hands like a flush or a straight or a complete bluff. By looking at the actions of our opponents we can get valuable information about their hands, and knowing this we can make better decisions. In the case above if we were holding a single pair it would probably be best to fold straight away, unless we are very certain they are bluffing.