Keyword difficulty (KD%) is a metric that measures the SEO effort it would take for your content to rank organically in the top 10 results on Google for a certain keyword. 

The keyword difficulty score is a percentage from 0 to 100, and the higher the percentage, the more difficult Semrush predicts it would be to rank for. Keyword difficulty calculation is based on a variety of factors such as the median number of referring domains pointing to the URLs that are ranking, the median ratio of dofollow/nofollow links pointing to these, the median authority score of the domains that are ranking, and the SERP-related qualities of the keyword itself. 

In general, there are 6 levels of how difficult it is to rank for a specific keyword:

• 0-14 = Very easy. These are the best opportunities to start ranking new web pages on Google if you want to rank as soon as possible with the least amount of effort.
• 15-29 = Easy. These keywords have some competition but are still achievable, even if your domain is new. To be able to rank for these, you’ll need to produce quality content focused on the keyword’s intent. 
• 30-49 = Possible. For these, you’ll need quality, well-structured content that is properly optimized for your target keywords.
• 50-69 = Difficult. To compete for these keywords, you’ll need some quality backlinks along with producing well-structured and optimized content.
• 70-84 = Hard. Winning these keywords will require a lot of effort in terms of getting better quality backlinks to support your well-optimized and unique content.
• 85-100 = Very hard. To compete for these, you’ll need to put a lot of effort not only into producing high-quality content and link building but also into on-page SEO and content promotion.

To learn more about the Keyword Difficulty score, check out this blog post. Or, download a detailed PDF all about Keyword Difficulty here.