The Messaging Tray is the bottom bar in GNOME Shell which contains indicators and message notifications.
The Messaging Tray is listed as one of the features of GNOME 3 at gnome.org, where a video of it is also available.

(Screenshot from www.gnome.org)
The Messaging Tray can either be made visible by moving the mouse cursor to the lower-right side of the screen in the normal desktop view, or by entering the Activities view.