HTML

HTML is the computer language a browser uses to display a website. Every website, without exception, is written in HTML.
HTML stands for HyperText Markup Language. It is a set of tags and conventions your browser can read, and that HTML is also called the source code of your website.
Contrary to what many people think, HTML is not a programming language but a markup language. All it does is describe the content of your page. You cannot use HTML to do calculations, for example.

Learning HTML is a good idea

You have not needed to know HTML to build a nice website for a long time. Systems such as WordPress, Joomla and Drupal make it very easy to build a site yourself without any HTML knowledge.
For SEO, though, knowing HTML does matter. You do not have to be an expert, but solid basic knowledge is a good idea. As with any computer language, there are several ways to get the same result, and some are better than others. Google understands the better ways more easily, and that gives you a real advantage.

HTML basics

HTML follows a fixed structure. Below is an example of an HTML page as you will often come across it.

<!doctype html>
<html lang="en">
	<head>
	    <meta charset="utf-8">
	    <title>My title</title>
	    <meta name="description" content="My description">
	    <link rel="stylesheet" href="/my.stylesheet.css">
	    <script defer src="/my.javascript.js"></script>
	</head>
	<body>
	    <h1>My first heading</h1>
	    <p>My text</p>
	</body>
</html>

What these HTML tags mean

<!doctype html> says the page is written in HTML5, the current standard
<head> holds meta information such as the title, the description and references to your stylesheet and JavaScript files
<meta charset="utf-8"> says your page uses UTF-8 text encoding. You usually do not need to know what that means, but you do need it in your page.
<title>My title</title> sets the title of the page
<meta name="description" content="My description">. The meta description tag holds the description of your page. Google often uses this description in its search results.
<link rel="stylesheet" href="/my.stylesheet.css"> Stylesheets control how your page looks.
<script defer src="/my.javascript.js"></script> JavaScript makes your page interactive. Use the 'defer' attribute so the page does not have to wait until the JavaScript file has fully loaded and run
<body> The actual content of your page goes in the body tag