Keyword difficulty

Comes from third-party SEO and creator tools rather than from YouTube, which publishes nothing of the kind. Each vendor builds the score from its own mix of inputs. The same term can come back easy in one tool and hard in another. Use it to sort a long list quickly. Then open the actual results page and look at who is ranking, how big they are, how old the videos are, and whether anything there answers the query. A high difficulty score on a query your audience keeps asking is still worth answering, because you are making the video for them either way.