Browsers were originally created to share text documents, primarilly between Universities and research groups. Imagine having to mail notes accross the country when you need to collaborate with co-workers.
To help make the text readable, simple things like space between paragraphs, highlights, and headlines are needed. Think about all the things you do in a Microsoft Word document. Bold text, margins on the left and right of the screen. With text in the browser that we share, those details need to be communicated to your computer, and we use HTML for that.
For example, when you click bold in the toolbar for Word, it will darken the text and make it fatter. To do that in an web document, you would wrap your text in special tags that the browser can understand. It looks like this:
<b> Hi Mom! </b>