Alt+F4 is a keyboard shortcut commonly used in Windows operating systems to close the currently active window or program. When you press the "Alt" key and the "F4" key simultaneously, it sends a signal to the operating system to close the active application or window. This shortcut can be useful for quickly closing programs without having to use the mouse to click on the close button. However, it's important to be cautious when using this shortcut, especially if you have unsaved work in the application you're closing, as it will typically close the program without prompting to save changes.