If you click on the new SubProject Credits link on your main Free-DC page near the top it should all hopefully become clear.
It's simply the number of subprojects in which you have crunched more than 10k. I was confused by the use of 'sub' too at first until I saw the new page.
There are quite a few Mega Milestones that are unexplained and you only tend to find out about them when you hit them! And even then it's not always obvious why you got them!
Bok has done all the work on this recently in part to compensate for issues with some projects becoming subprojects of the new Citizen Science Grid.
Hope that helps