In a superheterodyne receiver, a stage before the IF amplifier has a variable capacitor in parallel with a trimmer capacitor and an inductance. The variable capacitor is for:

In a superheterodyne receiver, the stage immediately preceding the Intermediate Frequency (IF) amplifier is the mixer. The mixer requires two inputs: the incoming RF signal and a signal from the Local Oscillator (LO). The LC circuit (inductance and capacitors) described here belongs to the local oscillator. The main variable capacitor is adjusted by the tuning dial to change the frequency of the Local Oscillator (LO), which in turn determines which incoming RF signal gets converted to the fixed IF. The trimmer capacitor is used for fine-tuning or alignment during manufacturing. It is not used for the BFO, which operates much later in the receiver chain at the detector stage.