Pressing Alt + F4 is a quick keyboard shortcut that closes the currently active window or application. If no applications are open, using Alt + F4 will bring up the shutdown dialog, allowing you to shut down, restart, or put your computer to sleep. This shortcut is particularly useful for quickly exiting programs or when a mouse is not functioning properly. However, it's important to save any open work before using Alt + F4 to avoid losing unsaved data.