Imagine that your prostate is an orange - the peel is the outer covering of the prostate and then the nerve bundles wrapping around the outer covering. The idea of nerve-sparing RP is to remove the orange while leaving the peel intact. There are two kinds of positive margin:
1. they discover after the op that the cancer had already got into the inner side of the peel
2. they discover that the orange isn't quite intact - a little bit was left behind
In case 1, a) the cancer cells have reached the peel but that bit of peel is now in a petrie dish in a lab or b) the cancer cells have reached the peel but may have been obliterated anyway by the heat / cutting / slashing. In case 2, that tiny bit of flesh that is missing from the orange might not have had any cancer cells in it anyway (because your whole prostate isn't cancerous) and / or may not have been left behind at all ... just because it isn't on the orange doesn't mean that it isn't in tiny ripped pieces on the surgeon's blade.
Neither scenario guarantees that there will be a recurrence although it is more likely in case 1 than in case 2 - data suggests that there is hardly any increased risk at all for case 2 over men who did not have a positive margin. Your surgeon has told you that you are in case 2 group.