A window manager is a software utility found in most GUI-based software and applications that manages the overall alignment and layout of graphical windows. Window manager defines and controls the appearance and positioning of application interface windows.
Techopedia explains Window Manager
Generally, window managers work in collaboration with the desktop environment, logical graphical system and the underlying hardware to operate and manipulate graphical windows. These windows can be for the operating system or a software program/application. Some of the features window managers provide are the ability to minimize, maximize and close opened windows. The placement, effects, colors and transitioning may also be part of window managers, although this will depend on the underlying operating system, graphical libraries and system in use. Windows Explorer is a component in the Windows OS that incorporates and manages the window manager's tasks and features.