"No follow" means that a backlink has an html tag like the following:
<a href=”http://www.hello.com/” rel=”nofollow”>Link Text</a>
The following part is the "no follow" tag:
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.
Being "no follow" doesn't mean that the backlink is harmful or should be disavowed. They can still be valuable if they lead to referral traffic from people clicking on the link.