If you are working in Microsoft Windows operating system, "BSOD" (Blue Screen of Death) is a very popular term. It represents an exception or a fatal system crash. There could be multiple reasons (such as a general hardware failure or a process unexpectedly terminated) behind a BSOD crash.
BSOD is there since Windows 3.0 and that was always represented in a blue background. That's why it says "Blue Screen of Death". But now in Windows 11, Microsoft is probably changing the background color to black.