A dofollow link is the default state of a hyperlink: one with no rel="nofollow", rel="sponsored", or rel="ugc" attribute added to it. Search engines follow the link and can pass ranking value from the linking page to the destination, which is what makes a dofollow backlink from a reliable, authoritative site genuinely valuable.
There's technically no "dofollow" HTML attribute to add; it's the absence of a nofollow-family attribute that makes a link dofollow by default. That's why link building conversations focus on nofollow rather than dofollow: the useful lever is deciding which outbound links deserve to pass authority and which don't. A site linking out to an untrustworthy or unrelated destination, an ad, a user comment, sponsored content, should mark that link nofollow, sponsored, or ugc as appropriate, leaving genuine dofollow links for sources actually worth vouching for.
