The parameter $stylesheet is the name of the stylesheet, relative to the theme root. It also accepts an array of stylesheets. It is optional and defaults to 'editor-style.css'.
This function automatically adds another stylesheet with -rtl prefix, e.g. editor-style-rtl.css. If that file doesn't exist, it is removed before adding the stylesheet(s) to TinyMCE. If an array of stylesheets is passed to add_editor_style(), RTL is only added for the first stylesheet.
Since version 3.4 the TinyMCE body has .rtl CSS class. It is a better option to use that class and add any RTL styles to the main stylesheet.