HARO link building means responding to journalist queries — requests for expert commentary or data on a specific topic — with a genuinely useful pitch, in hopes of being quoted (and linked) in the resulting article. The platform brand has changed hands (HARO was acquired and relaunched under Featured.com after previously being folded into Connectively), but the core mechanic is the same, and similar journalist-source platforms exist alongside it, including Qwoted, SourceBottle, and others worth monitoring for relevant queries.
The link isn't guaranteed — journalists choose which pitches to use, and not every published quote includes a link back to the source's site. But when it works, you get an editorial citation from a real publication, earned purely because your expertise was genuinely useful for that story.