A good explanation from RAV1D but don't worry. If you were to accidentally press the F8 key right now, absolutely nothing would happen. Getting into Safe Mode requires the F8 key to be pressed during the computer's boot sequence (i.e. before the Windows screen loads), so it's a very deliberate action and not one you could easily do by accident.
However, if you do find that some suggests using Safe Mode to help fix a problem, it's nothing to worry about. You simply restart your computer to bring everything back to normal.
Chris