The Hand Tool is just what it looks like: a little hand that you use to click and drag on your scene to move it around without using the scrollbars to the right and bottom of the work area. (I'll refrain from any puns about it "giving you a hand" or being "handy".) This is especially useful when you're working with a large Flash movie, or simply zoomed in close to a work area and need to scoot around a little bit to focus on different areas.