Single phase rectifier
The secondary voltage arrows are confusing this. 
As mentioned in the comments the input and output will be AC. Every 1/2 cycle one of the two diodes will be forward biased and thus will permit conduction 

When Vin is positive D1 will be reverse bias (note the dot of the XFMR), but D2 will be forward bias. This facilitates current flowing around the bottom 1/2 of the output stage 

When Vin is negative D2 will be reverse bias but D1 will be forward bias. This facilitates current flowing around the top 1/2 of the output stage. 

The result is the load always see's positive current flow 







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