All musical instruments produce sound by vibration of something – a string, a membrane or a column of air. A good musical tone is produced when the vibrations have the right frequency (pitch) and right amplitude (loudness).
When a musical instrument is out of tune, it fails to produce sound of the right frequency and an unpleasant sound is produced. These instruments are tuned by making some adjustments. In stringed instruments, the strings (wires) are retuned by tightening or loosening them so that they vibrate with the right frequency when plucked. Similarly, in case of percussion instruments, like tabla, the tension or tautness of the membrane can be adjusted to produce the right sound when struck by hand.