The plain, unencrypted way a browser fetches a web page, which means anyone in between can read it.
Like sending your message on a postcard: every hand it passes through can read it.
A small retailer's shop ran a plug-in that still called an HTTP address for product images. Browsers blocked the images as mixed content, so the shop showed empty boxes where products should be, and the owner spent a weekend thinking the store was broken.
Managed Hosting keeps plain HTTP doing one job: redirecting to HTTPS. Domain Security keeps the DNS pointing at the right host so those redirects land in the right place.
Example scenario: A three-person accounting firm had an old HTTP link printed on business cards from years earlier. The link still worked because the site redirected HTTP to HTTPS automatically. Nobody had to reprint anything, and every visitor landed on the secure page.
If my site redirects to HTTPS, is HTTP still a risk?
Very little. The first request goes over HTTP before the redirect, so publishing an HSTS header tells browsers to skip even that. Your host can turn it on.

Every WILDCARD card exists to solve one named problem. Find yours.