Numb is an adjective that describes a lack of sensation. After skiing all day, your toes might be numb from the cold. You'd feel numb for hours after hearing that your favorite band broke up.
People can feel numb in terms of their feelings and emotions. For example, if you've had your heart broken lots of times, you start to become numb — the breakups don't upset you so much. Numb can also describe being so frightened that you can't move, like being numb with fear when you saw the bear near your campsite. Numb can also mean "to make insensitive," like when the dentist numbs your gums so you won't feel any pain.