If you are in a hand, and you lose or win, and after the hand, you are wondering if your thinking was correct, or did you just get lucky/unlucky.
You put your cards in. You put the flop cards in. Now, you have to put your oponent on a range. OR!!! (since you will know your opponent cards after the hand is done) you use the actual cards.
So now you chose to compare your assumed range of your opponent, or the actual cards. It is tedious and a lot of work and time consuming, but this is what you do off the table to get better when in a game.
I hope I am explaining this clearly but if you have any questions, please do not hesitate to ask.