in the keyboard, What is scroll lock for?

It is an artifact from an earlier time and not really used for much these days. I think there are a couple of utilities that you can get to use it and you can reassign it on many keyboards. I never did figure out how to use it in DOS, but it had something to do with controlling the scroll function in DOS. Check here for more info than you will probably ever want to know about it:

http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=Scroll+lock&btnG=Google+Search

Have only ever used it in Excel. If it's turned on then using the up/down arrow keys will scroll and not move from cell to cell.

That enables you to retain the active cell and navigate totally with keystrokes; using Enter and Tab to move the active cell down and across or up and back (if the shift key is depressed as well).

;)