The kazoo is a simple musical instrument that adds a "buzzing" timbral quality to a player's voice when you hum into it.
While humming is the term typically used to describe the technique required to play a kazoo, a more accurate term would be singing into the kazoo. A person can hum with their lips completely closed. Humming with your lips closed around the mouthpiece of the kazoo will not change the sound - you must vocalize or "sing" in order for the kazoo to make any sound. Many people will struggle with getting any sound from a kazoo when instructed to hum. But when instructed to speak "do, do, do..." into the kazoo mouthpiece, the 'hard' vocalization makes a more effective sound.