What's a Musou? Well, Musou is a subgenre of video games, specifically those derived from the Dynasty Warriors franchise by Koei Tecmo.
Not all Musous are necessarily made by Koei Tecmo (although a lot are), and there's a surprising amount of diversity across the genre.

what exactly qualifies as a musou?

To be entirely honest, I'm not exactly sure myself; that's a big part of why I want to play all of them, to figure out for myself what makes them so unique compared to other hack & slash/beat-em-up games.
The only exceptionally stand-out aspects of Musous are their notoriously high number of enemies on screen, and simple, combo-string based combat; and even those aren't even always a necessity!