The language web pages are written in; it tells a browser what is a heading, a paragraph, a link or an image.
Like the outline of a document: it marks which line is the title, which is a paragraph and which is a picture, before any styling.
A four-person consultancy pasted a Word document into their site editor. The page came out as one giant block with no headings, invisible to search engines and unreadable on a phone. A simple HTML structure would have put them on the first page for their own name.
Managed Hosting runs the site. Email Signatures handles the one piece of HTML every business sends daily: a signature that renders the same in every mail client.
Example scenario: A small physiotherapy clinic's new website looked beautiful on the designer's screen and broken on every phone. The HTML had been exported from a design tool with fixed widths. Rebuilding the templates with proper structure made the site readable on mobile, where most of their bookings came from.
Do I need to learn HTML to manage my website?
No. A CMS handles it. Knowing that headings, links and image descriptions exist helps you ask for the right things from whoever builds it.

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