I was reading a thread and came across a post I didn't understand. What does he mean by ranges exactly?

Play 10k+ hands of poker
Analyze 5+ big winning hands (75BB+) and write down the ranges from both perspectives
Analyze 5+ big losing hands (75BB+) and write down the ranges from both perspectives
Analyze 5+ hands where I called a 3bet in position and write down the ranges from both perspectives
Analyze 5+ hands where I 3bet the button from the blinds and write down the ranges from both perspectives
Analyze 5+ hands where I played 77-JJ without initiative and write down the ranges from both perspectives
Analyze 5+ hands where I 3bet AK OOP and write down the ranges from both perspectives
Analyze 5+hands where I played the turn and river HU OOP in a single raised pot with initiative and write down the ranges from both perspectives
Analyze 5+ hands where I played the turn and river HU IP in a single raised pot without initiaitve and write down the ranges from both perspectives

A range is the possible hands a player can have.

For example. When i raise UTG in a 6max game, my range may be:
55+, AT+,KQ+,78s+,QTs+

Now everyone folds except the big blind. His range might for example be:
22-TT, AJ-AQ,KQ,A7s-AJs,45s-QJs,68s-KJs (Better cards he would 3bet)

Of course, he doesnt KNOW what our range is, and we dont KNOW what his range is. HM/PT stats in combination with notes help us make an estimate of what the other players range is. So if above is what we estimate for his range, when the flop comes, we can reason how well that its his range. In this case for example, a Q on the flop will make him many top pair hands. Kings much less so if he reraises AK preflop. Lower cards he is likely to pair with a suited connector or a suited A.

Now every time he makes an action, you get more information and thus can reduce the range of hands that he could have. On the river, often only a few possible hands are remaining. Having a very good understanding of this will allow you to make better decisions.

And that is what the guy in this post means to do. In order to train himself, he wants to analyse hands and write down what he thinks the range of his opponent is at every action in the hand and what his opponent might expect his range to be.

+1 to Arjen.

To become a Power Ranger, you'd write down every combo pre-flop, and then write down the most likely combos that bet (or check) the flop, until you arrive at a very small number of combos on the river.
If you lose a big pot and your hand makes sense, but so does villain's, then it's basically a cooler. If, however, you find yourself thinking "I have 30 combos that value-bet this river, but villain only ever calls with 3 and they all beat me", it's often a sign that you made a mistake somewhere along the line.

Can you please post the link of the thread?

Thanks!