This option is very similar to a word processing application like Microsoft Word. You can use the buttons along the top of the Visual editor to style your content, add horizontal division lines, links etc.
As you edit, Save often and use the Preview button to see how your page will look once published.
Note: If instead of your content, you see code that looks like the picture to the left, it is likely that you are looking at the Text editor instead of the Visual editor. Switch back by clicking on the tab that says Visual, and you should see the content that you are expecting.
The Text editor is HTML. Using the Text editor can lead to changes that you are not expecting if you do not know HTML. If you do not know HTML, stick to the Visual editor.