I found this out from a random banner on Shopify support page while trying to figure out why my store’s email had stopped working so I figured I’d signal boost it.
Hooks are HTTP requests Shopify sends out when an order’s made that include order information. They’re sent to your servers, and let you do all sort of third party implementations/integrations (like deliver online registered software and ebooks, for instance). It looks like Shopify will be requiring all hooks to use HTTPS URLs by January 1st 2019. I have mixed feeling about our brave new SSL-only world, but chucking customer information around the network unencrypted is probably a bad idea, so get thee to shelling out $8 for the cheapest SSL cert you can and half that to the freelancer who will install it for you.