In this figure the transformer secondary is center tapped and the tap connects to ground. The top end of the secondary connects to the anode of D1 and the bottom of the winding connects to the anode of D2. The cathodes of the two diodes connect together and go through a resistor, labeled RL, to ground. There are two terminals one to each end of RL. The wave on the right is a sine wave which has been made all positive. The negative half cycles have been flipped over to be positive.