Voltage Doubler PWM Circuit
Your 555 timer produces a 0-12V square wave on it's OUT pin.  The circuit operates in two states.
First, with OUT at 0V, the diode on the left is forward biased and allows the 10uF capacitor to charge to 12V.  If it is not already charged above 12V, the 50uF capacitor will also charge.
Then when OUT goes to 12V, the now charged capacitor is placed in series with it, producing 24V on the other end of the capacitor.  This reverse biases the diode on the left, and forward biases the diode on the right if it was not already, charging the 50uF capacitor to 24V.
This output is controlled by the optoisolator gate driver.