A reed is a woody plant with long, slender leaves. Reeds need a lot of moisture, and they usually grow in or near a marsh.
The stalk of a reed plant is also called a reed, and it's from this stiff, wood-like material that reeds for certain musical instruments were first made. Many reed instruments, including the clarinet, still use reeds made from reed — specifically, a bamboo-like reed that is native to marshy Mediterranean areas. When a musician blows across her mouthpiece, the reed vibrates to make the sound.