Backlinks are links from 3rd party websites to your content. Each website has a unique domain. As a single domain can provide numerous backlinks it is important to have an understanding of how many unique domains the backlinks are coming from.
gShift provides this count within their app, reports and dashboards.
Example of Unique Domains:
List of Backlinks to www.example.com/awesome-website
Total Backlink Count is - 10
Unique Domain Count is - 8