The presentation is divided in pages, eventually grouped in chapters.
To navigate to the next page, you can press the bottom cursor key, or click the button. For the previous page it is the top cursor key, or button. The same way you can navigate to the beginning of the previous/next chapter with the left/right cursor key or / buttons. (a page which doesn't belong to a chapter is treated as a chapter of one single page)
Now, go to the next page !

Expo generates automatically the table of contents from the list of chapters and pages. You can display it by pressing the 't' key, or clicking on the button. Come back to the page you were the same way, 't' key or button.
From the ToC you can also jump directly to another page by clicking on its title.
The number of the current page is also displayed at the bottom of each page.

A page can contain whatever you can code with HTML and CSS. Graphics for example:

Or, formatted text...

You can also make your presentation dynamic or interactive if you know a little of JavaScript.

Expo is a single HTML page, ready to use without installing anything, easy to edit, share and version control.
To learn how to build your presentation using Expo, refer to this article.