HTML attribute very similar to the nofollow value . In this case it is used to indicate to googlebot (but not other search engines) that, although it can take the tagged link into account and follow it, it should not index it. It is generally used to prevent access to those contents of a website that have had to be duplicated. Thus, you can choose which version you prefer for publication from the search results.
This attribute does not fully ensure its effectiveness, since from other sites it could be linking without tags to the same page.