Keyword difficulty (KD) is a score, usually from 0 to 100, that estimates how hard it would be to rank on page one for a keyword. It's mostly based on how strong the pages that rank today are, especially how many quality links point to them.
Why it matters
A new or small website can spend months on a keyword where every result is a big, well-linked site, and never break through. Easier keywords bring results sooner and build the authority you need for harder ones.
Example
"Coffee" is extremely competitive. "Best coffee grinder for pour over under $100" is much easier, and the people searching it are closer to buying.
What to do
Each tool calculates difficulty differently, so compare keywords within one tool rather than across tools. Start with lower-difficulty, relevant keywords, and plan harder ones for later.