Spam Score
Spam Score is a Moz metric: the percentage of sites with features similar to a given site that Moz has found to be penalized or banned by Google.
Updated , every quotation and figure read in its source that day.
Spam Score is a risk indicator from Moz's Link Explorer, shown as a percentage. It does not say that a site is spam; it says how often sites that look like it, on a set of measurable features, have been penalized or banned by Google.
How Moz calculates Spam Score
Moz defines it as "the percentage of sites with similar features to the site you're researching which we've found to be penalized or banned by Google". It comes from a machine learning model "which identified 27 common features among the millions of banned or penalized sites" Moz fed it. The score updates quarterly, and a site's score only changes when Moz's crawlers detect changes to those 27 signals (Moz Help Center).
Reading low, medium and high
Moz groups scores into three bands: 1% to 30% is low, 31% to 60% medium, and 61% to 100% high. It is careful about what a high score means: it "doesn't mean this site is necessarily spammy. It's a sign that you should do some more investigation into the quality and relevance of this site" (Moz).
What Spam Score is not
It is not a score of the sites linking to you: Moz answers that question directly, "No, it's not." It is not a Google metric, and Moz cannot see which links you have disavowed, because Google does not share that data (Moz). Like every correlation-based score, it flags patterns, not intent.
How to use it
Use it to sort a list of link prospects or linking domains, then look at the flagged ones by hand: the content, the traffic, the other sites they link to. Do not build a disavow file from the number alone. Google says that in most cases it "can assess which links to trust without additional guidance, so most sites will not need to use this tool" (Search Console Help).