I kind of think of it as the magic of creation. Hence "Source." But more generally it seems sort of like "Life Essence" as well, hence obtaining it from souls and whatnot. In the first game (D:OS), Source was corrupted, but I don't know if that was temporary or permanent.
Some guy on Reddit: "Sourcery is a special type of magic originating from 'The first garden' the domain of Astarte, godess of the source."
Take from that what you will. Given how central it is to the game, it probably would be nice to have some detailed lore on source very early on (or perhaps there is and I haven't found it.)
