As others have said. The dark section is where your newly created character starts (which varies from character to character, based on stats and culture), the light section is skill points gained since then, and the black section is the way you have to go to reach 100.
Note that there doesn't seem to be any progression towards a new skill point, but rather a flat percentage chance for an increase. This chance remains the same for the first roll as for the 600:th (after 599 failures). The percentage chance gets lower the higher the skill is, so while it might be al 10% chance to go from 5->6, it might be 0.1% from 99->100 (the percentages used here are not based on actual values, although it IS very hard to go form 99 to 100 in e.g. dodging and blocking, with a chance well below 1%).