Shruti boxes are a very simple drone instrument. They are often used to accompany Indian wind instruments, especially the flute. One side of the shruti box houses a bellows, and by pumping it with one of your hands, air flows into the reeds housed on the opposite side. Each hole where the brass reed is placed has a movable plug. By rotating these you can open and close the air flow. Opening the flow will make the reed sing and closing it will mute the sound. The shruti box probably originated from the harmonium. The advantage over the harmonium is that it's small and thus much lighter. In addition, the drone notes are fixed and so less prone to mistakes, especially by untrained players. Finally, a shruti box costs less, is easy to transport, and is easily tuneable.
The possible combinations or reeds of can produce hundreds of sounds. The male shruti has a deeper sound than the female. To play, open one or more reeds by pushing the toggle to the side, unlatch the integral bellows and unlatch the reeded side. It is essential that at least one reed be open when you pump the bellows.