To drone is to make a low, continuous noise that sounds like humming or buzzing. On summer nights, you might hear cicadas drone, and on snowy winter mornings, hear your neighbor's snow blower drone.
Like animals and machines, people drone too, their voices sounding dull and tedious: "When my history teacher drones on, it's hard to stay awake." Drone is also a noun, meaning the monotonous sound itself, like the drone of a boring lecture; or a male worker bee; or a person who's as unambitious as a worker bee. Finally, there are the drones that fly by remote control, doing surveillance, gathering weather information, and occasionally firing missiles for the military.