A resonant cavity consists of a volume enclosed by metal walls. At specific frequencies there exist resonances where energy is exchanged between the electric and magnetic fields. The situation is analogous to a pendulum where there is an energy balance between the kinetic and potential energy and the oscillation frequency is dependant upon the pendulum specifications.

In a cavity the lowest frequncy for which resonance occures is defined to be the fundamental. Higher order resonances can occur and each has a specific field structure within the cavity. There are an infinite number of discrete resonant frequencies, however only those excited by the frequency generating source will be present for heating purposes.

The cavity fields are closely linked with the currents on the inside surface of the cavity walls and the presence of any material enclosed within the volume. A particular resonant frequency will have its bandwith broadened depending on the extent to which losses are present in the walls and filling material.
Figure 5 - Resonant cavity modes
(University of Stuttgart,Phy)