This tool is called a "Borderless Window Tool".
As you may already expect it removes the border from any window and can it fullscreen if the user wants so.
This is nice for dual monitor setups, but might be also usable for single monitor setups.
The major problem with fullscreen gaming on dual monitor setups is that the window becomes minimized when you switch to the second screen or to another window.
A few games already support borderless windowed mode, but for those that don't you can use this tool.
With the help of this tool those days are gone and you can play on fullscreen and switch apps without minimizing your game.