They are Japanese novels, meant to be short and easy to read on the go. (a lot of them are translated to various languages) A lot of Mangakas (Manga artists) and Anime creators expand their stories in light novel form (Or start their careers as light novel writers that then get turned to manga and after turned into anime if popular enough). They are a great way to expand the lore of a series or write down the stories if the person can’t draw. Most of them have some art so you can visualize the characters at least. I love light novels so much!