What it sounds like, and the records to hear it on.
A tanpura is a drone string instrument. It is traditionally used in Indian music, either played by the singer or by someone else, perhaps a student. Tanpuras vary in size and number of strings. For singing, a four stringed tanpura is used either tuned to G-B for ladies or C-D for gents. The tapura plays the tonic (the sa) and either the fifth (pa) or fourth (ma) depending on the raag being rendered. It's sound should be on going. It gives a point .