Checkout & Registration Manager scans user-inserted HTML for non-secure (http) references to files, and validates that all links are secure (https) links. If non-secure references are found, you will see the following warning in Checkout & Registration Manager:

The HTML text you've entered for the head section contains one or more insecure (http) links. Checkout pages must contain only secure (https) links, or customers will see a security warning during checkout.
Please ensure that your HTML text contains only secure (https) links.

If you see this warning, we recommend scanning HTML you've entered for http links, and replacing these with secure https links. References to non-secure links will cause a customer’s browser to display a warning that the page is not secure, which can negatively impact conversion.

If you're referencing images via HTML, use the instructions in this Help Page to make sure that any images/files are referenced from a secure (https) directory.