A referring site (or a referrer) is the source from which a website came from. For example, a web user might find some recommendations for a restaurant on a good food guide website. If the website includes a link to the restaurant website then it is referring the user to the site. This link is referred to as an inbound link.
If you have are using Google analytics on your website, you can track to see where your visitors are coming from by clicking on traffic sources. This data is divided into direct traffic, referring sites, keywords etc. If you click on referring sites, you will be able to see all the sites that have directed web users to you and how many people they have referred.
In terms of Search Engine Optimisation (SEO), it is important to build the amount of inbound links to your site. Not only do they provide traffic to your site, but also improve your Page Rank which can improve your rankings within search engine results pages (SERPs).
