The Break key is a keyboard key whose original purpose was to immediately halt execution of a running a program. On modern computers, it has no default function. It is not commonly used, although any software may use it for its own purpose. The Break key may also be remapped with a program like AutoHotkey.
Funny, in my early PC days it was a very frequently used key. Lots of times programs would hang in a way that you couldn’t otherwise get out of them. It was also super easy to make a mistake in a DOS command or .bat file that would either run endlessly or for way longer than you intended, so being able to just stop execution was super useful.