When the blue input sine wave goes positive the green output sine wave goes positive. When the blue input sine wave goes negative, the green output sine wave goes positive. It converts the alternating current to direct current. You can see when the sine wave goes positive the current is flowing thru 2 of the 4 diodes and going thru the resistor. When the sine wave goes negative the current goes thru the other 2 diodes and flows the same direction thru the resistor.
