A "drone" is an effect or accompaniment that sounds continuously throughout a piece of music, grounding the composition, providing a sonic, and occasionally a spiritual foundation.  Drones appear in many forms of vocal music in many parts of the world, but they're almost ubiquitous South and Central Asia where Tibetan monks chant mantras and in Georgia, way up in the Caucasus Mountains, where polyphonic singing is all the rage -- and has been for centuries. 

Drones also make their mark by being built into many instruments, often giving musicians a chance to play a constant tone on one part of the instrument while the others are available for melody, like the sitar in India or the Scottish Highland Bagpipes. Other instruments, like Aboriginal Australian didgeridoo and the Indian tanpura, are functionally drone machines, with their entire purpose being to drone while other instruments improvise. (In class our droning is mainly vocal, though in our imagination we are didgeridoo geniuses.)