5 stars is only for books that are my favorites, things that had enough of an impact I'm still recommending it months later. I rarely use it

4 stars, kept me engaged the whole time, solid writing

3 stars is for a middle of the road book, it was fine.

2 stars is for a book I didn't like. Usually has grammatical issues or repetitive language, or for fiction, characters who don't seem distinct from one another.

1 star is a book I hated, or had serious issues with grammar, or blatant sexism, racism, or homophobia. Rarely use it.

I also add or remove stars for some things. Like if it wasn't my type of book but seems valuable for others to read it, like it gives an important perspective that I otherwise wouldn't be exposed to, I add a star. I recently gave one star for animal abuse. Like, the writing was good, but the asshole abuses animals and still gets to be a NYTimes bestseller? One star for you.