I get this message when i try to imbed my iframe on my shopify website.

You are seeing this because this <iframe> references a HTTP URL. For better results, use HTTPS at all times.

Can you help please?

HTTPS URLS indicate a higher safety standard than the usual HTTP URLS.
Various browsers and companies may encourage you to use https instead of http especially if money or private information is being transferred.

Here's a link to an explanation of HTTPS and secure site certificates:https://www.globalsign.com/en/ssl-information-center/what-is-an-ssl-certificate/

if the url is for your video, i think youtube offers https URLs