Section Editing allows a user to edit a sub-section of the page only. This makes it easier to change long documents. Section Editing is sometimes called Partial Page Editing.
If you have the rights to do it, each headline (usually only the first 3 levels) starts a new section. When a page is writable a small Edit
button appears at the end of the section. Using this button loads only this section into the editor.
In the Admin page, you can set up the maximum level of section editing, so that your page isn't flooded by “edit” buttons. See Maximum section edit level
in your admin page or maxseclevel in your local.php config file.