A "no follow" tag tells search engines that the backlink should not pass SEO power (ie. link juice) to search engines. Accordingly, backlinks with "no follow" tags don't pass link juice.
Having a "no follow" tag does not mean a link is spammy and should be disavowed. Whether a link should be disavowed depends on many factors and should not be decided based on any single factor or metric. This guide should help: