Rectification:

A rectifier diode passes current only in one direction under ideal operating circumstances. This makes it helpful for changing ac to dc.

Commonly speaking, whenever the cathode is negative with respect to the anode, the current flows; whenever the cathode is positive relative to the anode, the current does not flow. The constraints on this behavior are the forward breakover and avalanche voltages. Throughout a little less than half the cycle, the diode conducts, and for the duration of a little more than half the cycle, it does not conduct. This cuts off a little more than 50 percent of every ac cycle. Based on which way the diode is related in the circuit, either the positive portion or the negative portion of the ac cycle is blocked.