The scroll lock is a key (with an associated status light) on a computer keyboard. The original purpose of the scroll lock was that when it is turned on, it would pause text scrolling down the screen until it is turned off again. However, it only works on software that supports it, which most software today does not. While it was useful in the days of character interfaces, in a GUI environment (where such things as scrollbars are provided) it is much less useful. Programmers can however make their software respond to the key in whatever way they like, hence it need not be restricted to its original function.
On some UNIX systems, control-S is interpreted as scroll lock while control-Q unlocks it.