A klystron looks and works something like an organ pipe.

Organ Pipe

In an organ pipe:

• Blowing into the organ pipe produces a flow of air.
• Flowing air excites vibrations in the cavity of the whistle.
• The vibrations flow into the surrounding air as sound waves.

In a klystron:

• The electron gun produces a flow of electrons.
• The bunching cavities regulate the speed of the electrons so that they arrive in bunches at the output cavity.
• The bunches of electrons excite microwaves in the output cavity of the klystron.
• The microwaves flow into the waveguide , which transports them to the accelerator.
• The electrons are absorbed in the beam stop.