KD stands for "Keyword Difficulty" and represents an estimation of how hard it would be to rank on the 1st page of Google for a given keyword.
KD is measured on a scale from 1 to 100, with latter being the hardest to rank for.
We use our own unique algorithm to calculate KD.
This algorithm is entirely based on the backlink profiles of sites that rank on the 1st page of Google for a given keyword and doesn't take into account any "on page SEO" factors.
For more info on KD score, please, refer to this post on Ahrefs Blog.