Your code from the introductory tutorial or elsewhere includes some of HTML tags. The things with < and > around them.
Tags normally come in pairs and often have stuff between the pairs of tags. For example (don't put this into your page):
<h1>My main page</h1>
The opening tag and closing tag are the same apart from the slash. The content is text and the tags here identify it as a heading.
The two tags and the stuff between are known as an HTML element (the example above code and text all together makes an h1 element).
Elements with only one tag
Just to add confusion a few elements only have the opening tag with no content or closing tag. The image tag as used in the introductory tutorial is one example:
<img src="something.png" alt="an image">
Generally you need to make sure tags come in pairs and just remember that the exceptions are the ones with no content (img, br, input, link, meta and hr are the common ones).