Considering this game is closely related to the first DOS, if you play this one without having played the first one, you will be missing a little bit of background knowledge.
In my experience though, even in the first DOS, all we know is that "Source" is a type of supposedly evil magic that Sourcerers have access to. But that's about it, really. If an enemy throws a bunch of hostile spells at you, it makes little difference which one is classified as Source magic, which one is "normal" magic. Or maybe I've forgotten a lot, since it's been a long time since I last played the first DOS.
Only in DOS2 where we play as Sourcerers that we actually experience this distinction. The whole idea of having normal magic and special magic is supposed to be a new element, in terms of gameplay if not lore, and you just figure out as you go. Your characters have access to a bunch of special abilities that you have to unlock gradually. And I think the use of the blue dot symbol for Source on the ability icons makes it easy to realize what's going on.
EDIT: The very first NPC you run into at the start of the game explains to you that the collar on your neck prevents you from casting "one of those Source spells of yours". So tbh, I think the whole thing is obvious enough.
Look all I'm asking for is a book thats easy to find, or a blurb in the new journal that explains what source and source magic is, how it works, and why there are only certain people capable of casting it. I've heard players say things like "we are prisoners because we can use magic", or something along those lines over and over again, even in different reviews so clearly its not clear enough what the distinction is.
Its been stated numerous times DOS1 should not be mandatory to understand DOS2 by the devs, so adding more lore and explanations to the game are a good thing. Honestly, I don't think they did a great job of it there either. I went through most of the game thinking it was just really powerful magic or necromancy, not that had anything to do with soul manipulation.
If you've never played a Divinity game the word source means nothing to you. When a character says source spell, or source magic the person completely new to franchise just thinks magic, and thats it. The game does a great job showing why source is dangerous and feared, but it doesn't do a good job of explaining what source or source magic is early on, and considering source is so important to the entire setting it should be one of the first things explained to the player.