The Nofollow attribute is an instruction that can be inserted into the HTML code of a link to signal search engine crawlers not to follow the link and not to transfer any "link juice" or "link authority" to it.
Nofollow attributes are often used in situations where webmasters do not want to endorse the linking to a particular page, such as in the case of paid links or links in comments. It helps prevent the passing of PageRank to undesired or untrusted pages, thus preserving the integrity of the website.