Alt+F4 is a keyboard shortcut often used to close the currently-active window. For example, if you pressed the keyboard shortcut 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 keyboard shortcut
Alt+F4 in Microsoft Windows
While running any program in Microsoft Windows, if Alt+F4 is pressed, a command is issued to the computer to close 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 closes the window with no warning.
Related keyboard shortcuts and keys
Below are links to related keyboard shortcuts and individual key pages.