GNOME Panel is a highly configurable launcher and taskbar for GNOME. It forms a core part of the GNOME desktop.
It has been replaced in GNOME 3.x by default with GNOME Shell, which only works with the Mutter window manager. GNOME Panel serves as Fallback Mode when Mutter can not be executed, although it can still be activated if a user still wishes to use it as their default desktop.
Read more about GNOME Panel: Appearance, Customization