When you hit a tuning fork with a hammer, a sound is produced as the tines of the tuning fork set surrounding air particles into vibration. The sound produced by the tuning fork is not that loud. When this oscillating fork is mounted on a sound box, the box begins vibrating at the same natural frequency of the tuning fork.  If the dimension of the box is just right for resonance, an oscillation mode will be set up in the box.  This results in an higher sound intensity.