Alt+F4 is a shortcut key most often used to close the current active window. For example, if you pressed the shortcut key now while reading this page on your computer browser it would close the browser window and all open tabs.
If you want to close a tab or window open in a program but not close the complete program, use the Ctrl+F4 keyboard shortcut.
How to use the Alt+F4 shortcut key
Alt+F4 in Microsoft Windows
While running any program in Microsoft Windows, if Alt+F4 is pressed it closes the currently active program. If no programs are open or you are at the desktop pressing Alt+F4 opens the Shutdown window.
Unless the program has unsaved changes pressing Alt+F4 will close the window with no warning.
Related keyboard shortcuts
Below are links to related keyboard key and keyboard shortcut pages.