A manager controls a portion of a business. Managers who think of themselves as important may not be liked by those being managed.
A manager who works for a company that makes windows is a window manager.
Linux is an operating system, which manages the resources of a computer. The display of a computer is a resource. Users interact with Linux via windows on the display. A window manager controls windows on a Linux display. The window manager allows users to create, move, resize, and delete windows. Linux has many window managers to select from. KDE, Cinnamon, Gnome are three.