Referring websites lets you track which sites send you the most traffic as well as the highest quality traffic. For example, one website may send you lots of visitors but the Bounce Rate is 100% meaning they left your site without looking at any additional pages. Maybe another site doesn't send you as many visitors but the Average Page Views and Average Time on Site is much longer then other sites meaning the quality of the visitor is better since they engage more with your site.
Referring web pages is similar to referring websites in that it lets you track which web pages across all the websites send you the most traffic as well as the highest quality traffic. Web pages that send you lots of visitors but have a high Bounce Rate are probably not as high quality as a web page that sends you less visitors but they have a higher Average Page Views and Average Time on Site.
This graph shows the number of unique websites that send you traffic each Day, Hour or Minute. It's a great way to measure how diverse your traffic sources are and how your diversity changes over time.
