what is keyword difficulty in plain english?

A tool's guess at how hard it'd be to rank on page one for a keyword, mostly based on how many strong links the pages already ranking have. Ahrefs, Semrush and Moz each calculate it differently, so the same keyword can be 20 in one and 45 in another. Use it to compare keywords inside one tool, not as an absolute. Look at the actual results too. If page one is forums and thin pages, it's beatable whatever the score says.