Tuning Resonant Cavities
For us folks who've migrated from lower-frequency discrete LC resonators toward higher-frequency cavity resonator, the progression might be viewed this way:


The cavity might be viewed as a hollow cylinder (like a tin can) with a central post electrically attached to the base, and extending up almost to the can's lid. The small space between the top of the post and can lid is the capacitor, whose spacing can be tuned with a screw.

Alternative views:
A coaxial transmission line is shorted at one end. It is less than one-quarter wavelength long. At the open end, a discrete capacitor is added to tune to resonance.

A coaxial transmission line is shorted at both ends. It is less than one-half wavelength long. At the mid-point a capacitor is added to tune to resonance.