"Font weight" refers to how thick or thin the letters of a font appear. It determines the visual weight of the text, like being bold or light. Full definition
Things light temperature adjustment, font weight adjustment and more font options, more margin control and other enhanced features. (blog.the-ebook-reader.com)
Some of the fonts are darker than others but yeah the app doesn't give any option to change font weight. (blog.the-ebook-reader.com)
In my opinion, I also found that the promise of better font weights did not live up to the hype I'd heard in the way I had hoped. (ebookevangelist.com)