The circuit is a triangle waveform generator that uses as few parts as possible.
A 555 timer IC, 2 resistors and two capacitors make the triangle wave.
The IC is connected in a 50% duty-cycle astable square-wave oscillator circuit.
The square-wave output is fed from pin 3 of the IC to an RC shaping circuit.
When the 555’s square-wave output goes high, C2 begins to charge through R2 and the voltage across C2 increases as long as the output remains high. When the IC’s output goes low again, C2 begins to discharge through R2 reducing the voltage C2 as long the output remains low. The resulting waveform accross C2 takes the shape of a triangle. The best waveform linearity is obtained when R2 and C2 are made as large as possible. With the component values shown, the peak-to-peak output is 0.5 V at a frequency of about 200 Hz.