Left 555 is an astable that generates narrow trigger pulses for the 555 on the right which acts as a voltage controlled monostable using pin 5 to control pulse duration consequently the output waveform from the 555 on the right is a variable pulse width controlled by the voltage on pin 5. Note that the Pulse width to voltage relationship is not linear. If a current source is used to charge the timing capacitor instead of a resistor the PWM will be linearly related to control voltage on pin 5
