If you mean in a prologue, I'm not sure it applies. If in a scene in the body of the story, it's not that often that one has to explain the origins, etc. of magic; one has to be more concerned with their effects on the world (consequences of casting the spell, how it changes characters' interaction with the world; it can't make things too easy, or it becomes a simple answer and you lose all narrative tension).
To me, prologues should serve some purpose: either to intrigue the reader with something that may not be returned to until much further on in the narrative, to jolt the reader into an entirely different world by giving such a strange yet plausible scene that the reader is immediately aware "we're not in Kansas anymore, Toto", or, as said, to display a character in a particular light (that may not, necessarily, be the final view of the character; it may be used to mislead, with a different view of the character developing before the tale is over; though this takes considerable skill and planning to pull off). In other words, it's something that couldn't be done in the body of the book proper without damaging the overall structure of the narrative. And keep it brief. Long prologues can work, but they're notoriously tricky. Avoid infodump like the four horsemen all rolled into one. If the prologue isn't absolutely necessary to make the story work, it shouldn't be there.
