That was one of my favorite Magic SysRq keys. It's the Linux analog to Windows's Ctrl-Alt-Del.
It's the "Secure Access Key" (SAK): You press that key and it kills all programs hooked to the TTY (incl. X, in your case) and displays a proper login prompt so that you can know what you're about to login to was run by the system and not a clever malware trying to steal your password.
It's the "Secure Access Key" (SAK): You press that key and it kills all programs hooked to the TTY (incl. X, in your case) and displays a proper login prompt so that you can know what you're about to login to was run by the system and not a clever malware trying to steal your password.