Referring domain is an unique domain from which the backlink comes from. For example blackhatworld.com
From this referring domain you can have multiple backlinks from various (or even the same) pages. For example:
https://www.blackhatworld.com/backlink-number-1
https://www.blackhatworld.com/backlink-number-2
https://www.blackhatworld.com/bla-bla-bla
If these were all the backlinks the domain has, it would have 1 referring domain, blackhatworld.com and 3 backlinks.
Your primary focus should be on the referring domains, rather than backlinks.
Sometimes if you have a backlink in the footer of a website, it will show on every single page of that site. In that case you can have 1 referring domain and thousands of backlinks at the same time. And those backlinks won't be too hepful.