What is the basic function of tuning capacitors?

The "tuning capacitor" in a radio is involved in determining the frequency of an

oscillator somewhere in the radio. As that capacitance is varied, the frequency

of the oscillator changes.

If the radio device is a transmitter, then the frequency on which it's transmitting

changes.

If the device is a radio receiver, then the frequency of the internal "local oscillator"

determines the frequency that will be selected, out of everything coming down from

the antenna, and then processed, detected, amplified, and filtered, for your listening

pleasure.