Keyword Difficulty is how hard the keyword would be in order to rank in Google usually.

A tool I use is ahrefs. It had KD from 0-100. So a keyword with a 50 is much harder to rank than a keyword with 7.

Does that help?

None of the tools out there are any good because numbers and metrics can't tell you what you need to know.

You need to know how well the top 10-20 pages are set up
You need to know if the keywords they are ranking for are actually the ones that they are getting good anchor text links for
you need to be able to look and see if the links they are getting are hard to get

sometimes a powerful site will rank in a weak serp because there is not that much competition and yet the tools out there give the sites a high mark based on links that were for something else.

You are asking a VERY good question though. Its the number one thing people should learn and yet very few teach it.

The keyword SEO difficulty is a major keyword metric when doing a keyword research. The higher the value, the harder is to compete with others on the market using that keyword.

There are some major keyword research tools to measure difficulty like Semrush, Ahrefs, Kwfinder, Longtailpro, etc.. All tools measure score 0 - 100.

The important thing is to please don't rely on the tool. You have to evaluate SERP result manually. Because tool can't judge like the human brain.

Keyword Difficulty is a metric of a keyword which means how hard to rank in Google for a specific keyword. You can get data for keyword difficulty from many tools. I prefer Ahrefs, Keyword Revealer and KWFinder. The measuring score is 0 to 100. Difficulty 30 is easy to rank a keyword on google. For ahrefs you can choose 0-1 score for the easiest result.

I use Ahrefs. They're pricey but definitely worth it.