Section Editing allows 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.
Maximum section edit level in your admin page or maxseclevel in your local.php config file.edit button beside the header, see the plugin editsections