The toolbar toggle button is used to expand/collapse additional editor rows.
By default, WordPress will begin with the toolbar toggle button in the ‘closed’ state.
Clicking the toolbar toggle button (from the closed state) will expand the editor to show the additional rows.
Force Toggle Toolbar Open
Checking the option to “Force Toggle Toolbar” will make the content editor always load in the “open” state. Even if the editor was in the “closed” state the last time it was saved; it will still load in the “open” state the next time the editor is loaded.