An informal label for an ordinary link without relationship qualifiers such as nofollow or sponsored; not a special HTML attribute or a guarantee of ranking value.
Dofollow is informal language commonly used to distinguish an ordinary link from one qualified with values such as nofollow or sponsored. It is not a special attribute you need to add to make a standard HTML link work. A normal anchor with a valid destination can exist without those relationship qualifiers.
The label does not guarantee that a search engine will crawl the destination, index it or assign a particular ranking value to the link. Those are separate decisions and processes. A vendor selling “dofollow links” may be describing markup, but that description alone says nothing about the relevance, quality or legitimacy of the placement.
Focus on why the link exists. A useful editorial reference and a paid placement arranged to manipulate rankings are different situations even if their visible wording looks similar. Commercial relationships should be accurately qualified instead of hiding the relationship to obtain a supposedly more powerful link.
Takeaway: Treat dofollow as informal terminology, not a guarantee of authority transfer. Evaluate the actual source, destination and relationship before agreeing to a placement.
Example
A trade magazine references a manufacturer’s original comparison table because it helps explain an article. The reference is an ordinary link. A separate paid advertisement in the same publication needs appropriate qualification; calling it dofollow would not change the commercial arrangement.
What to check
Ask to see the actual page and link markup rather than relying on a sales label. Do not request removal of appropriate paid-link qualification as a condition of sponsorship. If someone promises a fixed ranking gain from an unqualified link, ask what evidence supports that claim.
For more detail, see Google outbound link qualification.