Refers to a panel at the top of the screen in which indicators and menu icons may be displayed in some desktop environments.
A "top bar" is a feature of many Linux desktop environments (DEs) such as Unity, GNOME, MATE.
Indicators, such as date and time, battery status on a laptop, applications running in the background that present notifications, may be displayed in a top bar. Menu items such as applications and settings may also be available there.
A generic term for this type of feature is a panel. A top bar is a panel that runs along the top of the screen. In some desktop environments, panels can be repositioned from the top to the bottom or vice versa, or otherwise moved or removed. They may also be configured to hide until the pointer moves into the space they occupy when visible or touches the edge of the display. Flexibility varies between DEs.
Related tag: panel