The current goes thru the upper left diode and down thru the resistor. Then thru the lower right diode. When voltage reverses the current goes thru the upper right diode and then the same direction down thru the resistor. Then passes thru the lower left diode. The current flows the same direction thru the resistor regardless to the direction it flows from the AC power supply. The capacitor charges to keep the changing voltage constant. The blue sine wave is the AC input signal and the green is the DC output signal
