As Nafor say, SPR stands for Stack to Pot Ratio. Its measured only on the flop and gives an indication of, how committed (or not) you are to the pot with a strong but not nutted hand like an overpair or top pair good kicker. SPR is defined by the effective stack. So if for instance the pot is 200, you have 2.800, and the opponent have 800, then the SPR is 800 / 200 = 4. Lets say flop is KJ5 rainbow, and you have AK. Then you should probably not plan on going anywhere, and the only consideration is, what is the best way to get all the chips in the middle. However if the opponent has 3.400, then you are the effective stack, and the SPR is 2.800 / 200 = 14. Which mean that now you probably dont want to play for all the chips with just one pair, because if the pot gets that big, then chances are high, that the opponent has one of the
hands, that beat you, like KJ or 55.