Referrer pages are the pages where people have found a link that leads to your website. For example perhaps you posted a link to an event on Facebook. That link then leads to your website. This would be a referrer page.
Referrer pages are important because they give you an indication of where it is worthwhile trying to get links to events and content on your website. If the page constantly refers significant numbers of people then it is obviously worthwhile trying to get more links to the website from that page.
These pages can also be successful in helping you to understand what type of information people are interested in. If they have found the link on another page then the chances are that the other page has a significant amount of information about the same or a related topic. This meant that it is good practise to visit the referring pages and try to figure out what makes them attractive and why they are linked to you.
Often finding the referring pages can be difficult. Their identity can be hidden by the method that they link to you. For example if they link to you from a site using https and your site isn’t https the header information is stripped out so you can’t see the referrer. For most sites this isn’t an issue since you’ll be able to see the referring site.