A light novel, in the context of Japanese publishing, is pretty much just a (usually short) book aimed at the same general audience demographics as manga/anime (shonen, shoujo, etc.). It is almost entirely prose with maybe a handful of illustrations sprinkled throughout the book, maybe at chapter beginnings or climactic scenes. Manga, on the other hand, is art-first and is fully illustrated the whole way through (see !manga@ani.social). A lot of anime series start their lives as light novels (Danmachi, the subject of this post, is one of them). They are then later adapted to anime format, similar to how many Western movies are adaptations of books (Jurassic Park, LOTR, etc.)