Hi - Dave here from Moz Support,
Spam Score in Links and Link Explorer represents the percentage of sites with similar features we've found to be penalized or banned by Google. Our machine learning model identified 27 common features among the millions of banned or penalized sites in the data we fed it.
You can read more about Spam Score on our Help Hub - including an awesome video that explains how to use the score as well as the factors we use to calculate the score.
Your Spam Score - This does not mean that your site is definitely spammy. The percentage represents a wide variety of potential signals ranging from content concerns to low authority metrics. Since this is based on correlation with penalization, rather than causation, the solution isn't necessarily to change these factors on your site, but it may be worthwhile to consider them. It is best to use this percentage figure to judge the quality of inbound links to your site, giving you a signal to help you determine which of those links needs some further investigation and, perhaps, even removal.
Another site's Spam Score - Again, this doesn't mean that these sites are spammy. This percentage represents a wide variety of potential signals ranging from content concerns to low authority metrics. Since this is just based on correlation with penalization, rather than causation, the solution isn't necessarily to disregard sites or disavow links with higher Spam Scores. Instead, we'd recommend using it as a guide for kick starting investigations. Be sure to check out a site's content and its relevance in linking back to you before disregarding or disavowing.
You can read more about Link Explorer metrics here
https://moz.com/help/link-explorer