Working of tunable filter
IC2B forms an inverting integrator hence, when its output is summed with the input signal (at the junction with C10), DC offset is removed but, because the integrator will also only partially attenuate low frequencies, low frequency AC signals are also attenuated. As the input frequency gets higher, there is less attenuation.

This is high pass filter action.

The 3 dB frequency of the high pass filter is controlled by RV6 in series with R21.

C12 and R22 act as a high-pass "stopper" to the integrator thus, the total circuit action is to provide a steeper spectral slope to the attenuation of low frequencies than that from an integrator with DC at its non-inverting input.

If in doubt, simulate it.