"A window manager is system software that controls the placement and appearance of windows within a windowing system in a graphical user interface."
sourceWindow managers render and organize the apps that are open on your screen. Resizing, minimizing, and moving windows around the screen are all things window managers do.
There are floating/stacking window managers & tiling window managers.
To learn more about a specific type of window manager, go to its page using the top bar.
bspwm is my favorite window manager. I like it because it is very configurable and easy to make look nice. A screenshot of my configuration of it can be found in the page on tiling window managers.