A standard telegraph key has a built-in knife switch that can be used to short the keys contacts. When the key was not in use, that switch was kept closed, so that a signal was continually sent. If that steady-state signal was interrupted, it indicated one of two things: either the operator was about to start transmitting, or something else had happened to break the connection -- such as hostiles cutting the telegraph line.
As the teletype? came into use, a key was created that would temporarily interrupt the communications line, allowing this practice to continue.
On personal computers, the break key is used in different ways by different programs, but usually involves some sense of changing the interaction the computer, such as switching between multiple login sessions or interrupting a modem connection.
See also SysRq, scroll lock.
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