Keyword difficulty
Keyword difficulty is a score estimating how hard it would be to rank on the first page for a term, based mostly on how strong the sites already ranking are. It is a tool's opinion, scaled zero to one hundred, not a measurement.
What the score is actually looking at
Mostly the strength of the pages currently ranking: how many other sites link to them, and how established those sites are. Some tools fold in a little more.
What it usually cannot see is whether those pages are any good. A term with a high difficulty score and five weak, out-of-date results is easier than the number suggests. A low-difficulty term where every result is excellent is harder.
How to use it
Compare it against your own site, not in the abstract. A difficulty of 30 is a reasonable target for an established site and a long project for a new one.
Scout pairs difficulty with what a term could realistically bring you, then queues the ones where the ratio is favourable. Starting with the easiest winnable terms and moving up is slower to feel exciting and faster to work.