How hard it is to rank on page 1 for a given keyword. Higher = harder.
Keyword difficulty (KD) is a 0-100 score estimating how competitive a keyword is. Tools (Ahrefs, SEMrush) calculate it from the strength of sites currently ranking. High KD = you'll need a strong site with lots of backlinks to compete. Low KD = even a new site can potentially rank with good content.
Targeting high-KD keywords on a new site is a waste of months. Start with lower-KD long-tail keywords and build authority. Graduate to harder keywords as your site earns trust (backlinks, time in index, topical depth). KD is how you know which keywords are in your reach.