A referring site, or referring domain, is a website that contains at least one link pointing towards your website. A list of referring sites is available via Google Analytics as part of the audience acquisition metrics. This means that before users came to your page they visited the referring site’s page and clicked on a backlink pointing to your site.
Referring sites are an important metric when it comes to organic ranking in search engines. It is used to measure a sites popularity in terms of backlinks.