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It's a conventional inverting Schmitt-trigger oscillator. R1,2,3 set the bias point for the non-inverting input of op-amp U1. When the inverting input is below the bias point, U1 output is high, which means the bias point is raised above half the supply voltage and defines an upper threshold. U1 output charges C1 via R5 until the inverting input voltage goes above the threshold. This causes U1 output to go low, so R1,2,3 now set a lower threshold and C1 discharges into U1 output, via R4 and R5 in parallel, towards the lower threshold. When that is reached, U1 output goes high again and the cycle repeats.
It makes the charge path into C1 different than the discharge path.
Here are two simulations, one with R4=750KΩ, and the other with it not in the circuit (R4=1GΩ). Note that with R4 open, the output is a square wave. With R4=750K, the duty cycle changes (as does the period).
Oscillator is an electronic circuit which produces a periodic electronic signal, sine wave or a square wave. Oscillators convert direct current from a power supply to an alternating current signal. Oscillator widely used in many electronic devices.