Hotlink protection is disabling external use of your resources. For instance,
if your website has many useful images or videos, others may try to serve their clients the URL of your resources instead of your own in order to save resources on their end.
Enabling hotlink protection will mean that the resource will only load if it is accesses locally, such as on your website.
After enabling hotlink protection your image would not load from other web sites. If you add a new subdomain please redo the hotlink protection again.