ICM is the Independent Chip Model. This model assigns a value in real currency to each chip based on stack sizes and the number of players in a given tournament. PokerTracker can use this model to calculate the value of your stack in the Game Currency of the tournament.
At the time of an all-in where we can calculate equity accurately we use the model's results instead of actual chip counts in our adjustments. So if you are in an all-in with an 80% chance to win in a $20 tournament and if you win you have a stack worth the equivalent of $10 and if you lose you are out of the tournament. This would yield an expected result of .8 * 10 + .2 * 0 = $8. However, that's never exactly what happens -- either you win the hand or lose the hand. So from this hand either you would be $8 under (green line below gold line) or $2 over (green line above gold line).
For each all-in situation where we can calculate we run these same numbers and that difference is what gives you the gap you're seeing.
However, given that it doesn't help you make any particular in game decisions in any paticulr hand, it's only going to be useful for reference purposes. If you want to get better at making push/fold decisions based on what ICM predicts, we recommend you take a look at our ICM Quiz in Tools -> ICM. That will give you a bunch of push/fold decisions and show you whether your decisions were long term winning or losing decisions based on the mathematics of the model.