This Full Wave rectifier has only two diodes. Each diode conducts during one half cycle. In the first half cycle, diode D1 is forward bias and the current flows into the load and returns through the centre tap of the transformer.
During the negative half cycle, diode D2 is in conduction. The current flows through the load in the same direction and returns through the centre tap. Thus the current is in the same direction through the load.
This means of rectifier has generally been replaced by the bridge rectifier as the centre tap is not always available.