The Tow-Thomas-Biquad is one of the popular state-variable filter topologies.
The benefit of these structures is the availability of more than one filter functions at the same time.
The Tow-Thomas filter contains three blocks: (1) A damped integrator, (2) an undamped integrator and (3) an inverter.
It provides three ouputs: At (1) a bandpass function, at (2) a lowpass function and at (3) an inverted lowpass function.
The basic function is as follows: Without the damping feedback resistor at (1) the loop of these three blocks will constitute an oscillator (two inverting intergators with a phase shift of -180deg at the desired oscillation frequency). That means: It is the purpose of the damping resistor to inhibit oscillations and to reduce the Q-factor from infinity (oscillation) to the desired lower value.
This filter topology is extensively described in all relevant textbooks and in many filter tutorials which are available in the Internet. Try a search.