It’s a form of interactive fiction that usually supplements a text based narrative with audio and visual media. Some are basically books as computer programs, most have background art, music, sound effects, character portraits, and the occasional illustration. Many have partial voiceovers in key moments. There are VNs with linear narratives, or some that follow more choose your own adventure style plots. There are also quite a few that get into a solid hybrid between narrative and incorporation of game elements.
I’d say the most common is the type that starts with common exposition and branches into one of several (mostly linear) “routes” based on a character selection. Romance/smut is super common, but there are lot of other genres as well. In a VN the text is usually the bulk of the game with shorter VNs being less than 10 hours or so, but I’ve seen long ones reported at 100+ depending on reading speed.
This site is kind of annoying to use, but if you want see how deep the rabbit hole goes https://vndb.org/
A lot of anime comes from VNs, but if we want to get into memes Nekopara, Fate, Stein’s Gate, Ace Attourney, Danganronpa, ect. are all big names you might have heard of that are or originated from VNs. I hope this cover the basics well enough. I feel like someone more into the genre could write an essay here.
Edit: I recently played VA-11 HALL-A which was an absolute joy
You need vocab and grammar to read, but you also need to read to actually be able to cement and retain vocab and grammar. I think it all needs to be grown in tandem.
