Referral traffic is when people come to your website from other sites, and not the search engines directly. Visits from clicking on a link on social media or on any other site counts as referral traffic in Google Analytics.
Media features, social media presence, guest blogging, partnerships, review sites, forums are just some of the places to get referral traffic from. To increase your marketing ROI, you should monitor what referral traffic sources bring revenue and double down on them.
Here's how referral traffic monitoring looks in Metrilo: