To lower is to move something downward. When you go to sleep at night, you lower your head onto your pillow (unless you are a horse; in that case, you don't lower anything and sleep standing up).
After cleaning your house's gutters, you'll climb down, lower the ladder, and put it away. After you've raised your hand in class and been called on, you will lower your hand again. Another way to lower something is to decrease the volume: "Will you please lower the sound on the TV? It's way too loud!" A less common meaning of lower is to scowl or glower, or to look stormy and dark.