HTML is a format for organizing information. In a website, HTML is the first real code you get from a web server any time you visit. It is the conductor and skeleton of everything that goes and can go into a web page. While there's a wide range of browsers and an impetus to improve experiences on the web, they need a standard. Otherwise all the browsers would behave differently, leaving developers hopeless to support all the browsers, and users hitting roadblocks because they prefer one browser over another. HTML, specifically 5, is the answer to all these problems. It's an evolving standard developed collaboratively by web industry professionals.
HTML governs whether something is a link and where that link points to. A link in HTML looks like this: