A backlink is a hyperlink or web link from an external website that points to your website or a specific piece of content on your website. The links are also called inbound links, and the external website is often called a referring domain.
Backlinks are relevant because users often find a site by following credible links and search engines like Bing and Google use backlinks to help form a decision about a site’s domain authority. A website’s authority is what search engines use to arrive at search placement for a given term. In short, the website (domain) authority is equivalent to the website’s strength.