IF in superheterodyne receiver
The RF amplifier passes a fairly broad band of frequencies.  The trimmer capacitor allows you to adjust the center of that band to match the desired station.

The oscillator is adjusted to cause the mixer to convert the desired frequency down to the IF frequency.  The IF amplifier stages have a much narrower passband than the RF stage, so they can provide the selectivity to allow the receiver to select a single station from the range that the RF amplifier passes.