Yes, that which one wishes to convey. Explaining how would require explaining the entire basic function of magic to you, I do not think you would understand it.

[He heaved a sigh and tried the simplify it the best he could. It was a lost art even among Thedas from his understanding, explaining it to one who likely had never seen a spell in their life was... difficult.]

The simplest way I can put it is that Veilfire is the memory of a flame from the realm from which I draw magic. Tis formed by spirits who have never experienced a true flame. [He held his hand to the flame and let it dance along the skin without pulling his hand back.] It gives off light and it cannot burn, but it has unique properties that those with the understanding can make use of.