I think "best" for font kerning is fairly self-explanatory. ;-) Best kerning provides the best experience, but at the cost of opening the document somewhat slower. Which is well worth it to me.
There's actually a built-in explanation if you hold on the option name. Holding on an option itself allows you to set it as the default.
For font hinting it's… difficult. In principle native should be better, but some fonts just aren't very good with native, so we default to auto. Auto is very good regardless. Some discussion can be found in this issue:
https://github.com/koreader/koreader...ment-333471966
Since E Ink screens tend to have a fairly reasonable DPI value, it's not as if hinting
off is bad either. It might even be preferable. The tl;dr is that from a sufficiently high DPI (let's say at least ~200) the differences between those options matter less and less. Just toggle between them and see which you prefer. And I imagine
off should be imperceptibly faster on first load.
Edit: btw, in case any of the terminology itself is unclear:
https://en.wikipedia.org/wiki/Font_hinting
https://en.wikipedia.org/wiki/Kerning