What is "snow-walker"

Snow-Walker is a trilogy written by author Catherine Fisher. The book is presumably set in the Dark Ages of a fictional region in Scandinavia, as the story often heavily refers to Norse mythology. The story is mainly about the sorceress Gudrun, who rules the Jarl's people through fear; the power struggle between her and her son, her very reflection and one weakness; and the small, motley band of outlaws who join him in the fight to restore the land to its rightful leader.