This is a term used to explain what happens in the secondary coil in a tesla coil. The secondary coil is actually a whole circuit, consisting of the coil and a capacitor. The capacitor in this case is formed between the discharge electrode and the ground itself.

This, of course forms an RC Circuit. Thus the circuit resonates, meaning that the electricity goes back and forth between the plates of the capacitor (the ground and the discharge electrode), through the coil.

The primary circuit is tuned such that it breaks down in phase with the secondary circuit, and thus adds to the voltage thats already ringing through the circuit.

This is called ferranti rise. The secondary circuit acts like a gong being struck, and each time, the voltage rises more and more.

The end result being obvious if you have ever seen a tesla coil.