What they are: Otherwise known as incoming, inbound or inward links, a backlink refers to the link on another website that links back to yours. 

Why they matter: A large number of quality backlinks (from relevant and credible websites) can indicate popularity of your site and help improve your search engine ranking. 

How to use them: You can't necessarily choose your backlinks, but you can analyze the ones you have. Your inbound links can give you a better perspective of who is taking interest in your website and what cultural or personal significance your site may have for them.

The bottom line: The quantity and quality of your backlinks may be out of your hands, but you can learn from them by taking a closer look at who is linking, and from what type of site. This can help you zero in on your testing program and tailor your site to an actively interested audience, leading to a better user experience and potentially, a higher conversion rate.