What is a referrer?

Under PUBLISH SETTINGS, there is a field called referrer.

This option talks about the ability to add website referral restrictions to your content. For example, if you own the website, you can setup a referrer to block sites that aren't from trying to grab and reuse your embed code. To continue the example, if you have a referrer restriction setup and is trying to steal your content, the player will be blocked on their site if they try and copy the code onto it.

Here is a walkthrough to better explain how to set this feature up.