To hum is to make a low, steady, singing sound with your mouth closed. If you don't know the words to a song on the radio, you can just hum along.
Other hums are based on the musical hum, making a similar low, constant sound. Bees hum, an electric fan hums, and your car idling quietly also hums. A figurative way to hum is to be busy and loud: "The cafe always seemed to hum with activity." The word hum is imitative (it sounds like the noise it describes), but its earliest form was hommen, in the 1500s, meaning "make a murmuring sound to cover embarrassment."