Openbox is a window manager, a type of graphical user interface (GUI) for the X Window System. This stacking window manager is free under a GNU General Public License. Part of the appeal of Openbox relative to other window managers is its speed, simplicity, and compliance with the Inter-Client Communication Conventions Manual (ICCCM) and Extended Window Manager Hints (EWMH).
Techopedia explains Openbox
Unlike similar window managers such as Blackbox, an original product on which Openbox is based, Openbox does not rely on proprietary code but is written completely in the C programming language. Like Blackbox, Openbox includes useful hotkey features. A set of dynamic menus called pipe menus also add to the versatility of the Openbox GUI.
Openbox is considered a "lightweight" GUI, and its unique build can make its functionality on operating systems - including Linux - more agile. The speed and design of Openbox allow it to run on older or obsolete equipment with slower processors.