I believe it's referring to small intercharacter typesetting details like tracking and kerning.

From the microtype package for latex:

"The microtype package provides a LaTeX interface to the micro-typographic extensions ... : most prominently, character protrusion and font expansion, furthermore the adjustment of interword spacing and additional kerning, as well as hyphenatable letterspacing (tracking) and the possibility to disable all or selected ligatures."

I'm using it as a shorthand for the features provided by LaTeX's microtype package, namely proper kerning, protrusion, and expansion:

https://ctan.org/pkg/microtype?lang=en