The tray icon shows stash activity and can indicate whether specific stashes are mounted. The menu gives quick access to mount or unmount your stashes. You can hide stashes from the menu by disabling their respective "Show in tray menu"-option and organize them in stash groups. You can globally control the tray icon and the menu in the Preferences. Unavailable stashes aren't shown in the tray menu.

Please note that recent versions of the Gnome Shell don't have a tray area and you might to have to install an extension for tray icons to be visible. The version provided by Ubuntu 17.10 and 18.04 already has the extension installed by default, so there are no further actions necessary.