In a frequency multiplier circuit, an inductance (L1) and a variable capacitor (C2) are connected in series between VCC+ and ground. The collector of a transistor is connected to a tap on L1. The purpose of the variable capacitor is to:

In a frequency multiplier, the amplifier stage intentionally generates a wide range of harmonics from the original input signal. To make use of this, the output circuit must isolate the specific harmonic frequency you want to transmit or process. The combination of the inductor (L1) and the variable capacitor (C2) forms a resonant tank circuit. By adjusting the variable capacitor, you change the resonant frequency of this LC circuit. The purpose of tuning C2 is to make the tank circuit resonate precisely at the desired harmonic, not the fundamental frequency applied to the base. If you tuned it to the base frequency, the circuit would simply act as a standard amplifier rather than a frequency multiplier.