Hi! So, it’s never specified in canon, beyond the ‘truth’ aspect of it. It’s always hinted that there’s something more to it, and I have my own take on it in ACOWAS. I’m not sure how much you’ve read, but Mor’s gift is “sovereignty,” a rather abstract concept that means she bestows honor and glory upon those whom she chooses. It comes from myth, including but not limited to the actual gift of the Morrigan in Irish myth and other sovereignty figures like Dame Ragnelle in Arthurian myth.
I hope you’re enjoying ACOWAS! Thanks for reading!