We often format a piece of written content by changing the way it looks. For example, we may increase a heading's font size so it looks prominent and important, or highlight the text with a bright color or use bold or italics for emphasis.
So how do you describe the importance of these big headings or imply expressions like emphasis to someone who cannot see? One of the most important rules when creating accessible content is to describe what the text means in terms of importance, not just how it looks. Leave how it looks to the template designer (and if you are the template designer, then leave it until the next chapter!).
So how do you describe what something is or what it means, rather than just how it looks? How can you make a browser, interpret that meaning correctly? This is what HTML does very well.