In music, a drone is a note which sounds all the time while a piece of music is played. Some folk instruments always make a drone when they are played: bagpipes and hurdy gurdies in European culture, sitars in Indian music, and many other instruments in Asian and African music. Sometimes more than one drone is heard (often two notes which are a fifth apart).
Music with drones has to have simple harmonies because it is not possible to modulate to different keys. Other things can make up for this, for example, Scottish bagpipe music has lots of little ornamental notes to make it interesting.
Some Western Composers liked to use a drone (especially one in fifths) to make it sound like bagpipes or other folk instruments. Bach, François Couperin and other Baroque composers often called such pieces "Musette" (the French for "bagpipes"). Sometimes they did this in orchestral music: Haydn used a drone in the last part of his Symphony No. 104 to accompany a folk tune.