Drones are among the oldest musical devices: bagpipes, the Indian tanpura and the Australian didgeridoo all use them. Melodies move over the fixed note, creating tension and release against it.
In ambient and dark ambient music, drones become the main material, slowly evolving in texture.
Using it in a prompt
Ask for "a low drone underneath" for meditation, suspense and sci-fi scenes, or "drone-based" for minimal ambient pieces.