URL only has nofollow incoming internal links
This means that the URL in question does not have any incoming followed links from internal URLs - only nofollow ones.
Why is this important?
The nofollow attribute can be added to an internal link to instruct search engine crawlers not to crawl the linked URL. This also means that indexing signals such as link equity (e.g. Google's PageRank) are not passed through the link to the linked URL.
If a given URL receives only nofollow links from all the internal URLs that link to it, that means it will no accumulate link equity, and as such would have no power to rank for search queries.
In some cases, it is entirely appropriate for a URL to have only nofollow links pointing at it (e.g. a user login page). As such, there is potentially nothing wrong with these URLs, Sitebulb is simply notifying you that they are there.
What does the Hint check?
This Hint will trigger for any internal URL which has some incoming nofollow links, but no incoming followed links.
Examples that trigger this Hint
The Hint would trigger for any internal URL that was found by the Sitebulb crawler during the crawl (i.e. not found in XML Sitemap, Google Analytics etc...), which has 0 followed incoming links from internal pages, but at least 1 nofollowed incoming link.
Why is this Hint marked 'Insight'?
This Hint is an 'Insight', which means there isn't necessarily any action that needs to be taken - the Hint is intended to alert your attention to something, rather than flagging up an issue that needs fixing. There are plenty of common situations where a URL is not expected to rank for anything, and/or you don't wish search engine crawlers to end up there.
However it is worth double checking that there are no such URLs that you actually want to be properly crawled and indexed. If there are, then the solution for this issue would be to establish where each URL should sit in the overall site architecture, and ensure that the links pointing at it are followed links.