A 0-100 score of a site's backlink authority. Higher = stronger in Google's eyes.
DR (Ahrefs) and DA (Moz) and equivalents are tools' estimates of a site's overall backlink strength. Not a Google metric, but a useful proxy. DR 0-20 = new or weak site. DR 40-60 = established. DR 70+ = authoritative. DR 90+ = NYT, Wikipedia, major publishers. Your DR determines how hard keywords you can realistically compete for.
DR is how you read the competitive landscape. Before writing content for a keyword, check the DR of sites currently ranking. If they're all 60+ and you're 25, pick a different keyword.