VIBRATION

 Answer 

In order to produce a single pitch the tuning fork has narrow tines, so its surface area is small, and since radiation to the surrounding air is weak, so is the efficiency of energy transfer.

Attaching it (the correct acoustics term is coupling it) to the table increases the surface area, and allows a greater amplification of the vibration. In other words, the table acts as a resonator and the sound becomes louder even though no other energy has been added.