The quick answer is, I don't know. I could attempt to speculate, but that would not be especially useful to you. It would be best to wait for Raze to return: if there's anything Raze doesn't know about the Divinity universe, it's probably not worth knowing.
A very simplistic (and potentially inaccurate) answer is that it's a powerful form of magic that permeates Rivellon. Its practitioners have been variously celebrated and condemned depending on the politics of the day.
I saw many references to the source
but some this Object
some mystical energy
and no where no direct answer
even collecting information does not know anything
no books no NPC dont answer
I think even the developers do not yet know what it is
But for me the history of the world without this knowledge is incomplete
In the second part of diviniti origin we a Source mages but no in first part no in second dont tall what meen "magic of the source"