To the left of the editor area is the side panel. If you would prefer that the side bar be on the right side, simply change the setting in the User Interface tab found in the → menu option. The side panel consists of a file browser, a function reference browser, and a bookmark browser, accessible by clicking on the corresponding sidebar notebook tab.
You may also customize the location of the sidebar notebook tab via User Interface tab found in the → menu option.
The file browser provides quick access to files and directories.
The function reference browser references CSS2, HTML, PHP, and Python functions with their syntax. Some of them provide dialogs to help you inserting them.
The bookmark browser provides access to previously marked positions in a file.