A link from another website to yours. Google treats it as a vote of confidence.
Backlinks remain one of the strongest ranking factors. Each backlink is a signal that someone else found your content valuable enough to link to. Quality matters more than quantity: one link from the New York Times is worth thousands from low-quality sites. Google's algorithm specifically weighs link quality, topical relevance, and the anchor text.
Quality content alone rarely ranks in competitive niches; you need backlinks too. Building them (via outreach, digital PR, guest posts, being linkworthy) is half the job of SEO. Without a backlink strategy, you're leaving rankings on the table.