A harp is a stringed musical instrument where the sounding board isn't under the strings like on a guitar, but is at right angles to them, in the base of the harp.
Something is sharp if it has a thin edge or point. Knives are usually sharp, as are the thorns on roses. You have to be careful around things that are sharp so that you do not cut and hurt yourself. Brokenglass has very sharp edges. The opposite of sharp is blunt.