Definition - What does Nofollow mean?
Nofollow is a specific tag in HTML that allows for manipulation of the status of some hyperlinks. With a nofollow command, the object in question is prevented from passing link authority. This tag also prevents the link from having an influence on certain search engine index algorithms.
Techopedia explains Nofollow
Nofollow is often used on sites that contain different kinds of user-generated content, or where "comment spam" may occur. Because of the nature of many blogs and posting pages, spammers have submitted large numbers of nonsensical comments that can affect a page or site. Even other kinds of more coherent comments could be characterized as comment spam, and can help or hurt a site based on several factors. Often, web site maintenance experts have to weed out legitimate comments from this flood of "black hat comment" traffic.
In recent years, Google experts spoke out about the nofollow command, illustrating its use in terms of Google search results. SEO experts have responded, some adding that nofollow can also be used to affect the counting of outbound links on metadata assessments. One way that some professionals explain the use of nofollow is that even though outbound link activity can show interest in a page or site, it may also lead to situations where search engine algorithms falsely flag the site as violating SEO principles of the search engine’s own philosophy; in these cases, it is in the site owner’s best interests to use nofollow and other tactics to enhance SEO relative to current search engine algorithms.