Hotlink protection prevents your media content from being used on other websites by hotlinking it, which consumes your bandwidth.
To enable it, open your Websites → Manage page, search for Hotlink Protection on the sidebar and click on it:
Then choose which types of files you would like to protect (or select * to choose all).
You should also specify where all the requests to your restricted files will be redirected. It can be either a specific page of your website or any other website.
Save the changes - your hotlink protection is successfully set up! You can edit it on the same page.
Additional resources:
There are other options to enable hotlink protection (via .htaccess, plugins, CDN). You can find them in Hotlinking: What Is It and Why Is It Bad?