Half-Wave & Full-Wave Rectification
These pictures show the addition of a rectifier diode - a device which only allows
current to pass in one direction.
In the top pictures, the negative half of the AC cycle has been removed completely. Although now DC, this supply is very 'rough'.
In the lower pictures, a special 'centre-tapped' transformer is used. Here, by using two rectifier diodes, both halves of the AC wave can be used - which produces the waveform shown. This is much easier to 'smooth' into pure DC.
Incidentally, these waveforms show why it is important to set the centreline with the AC - GND - DC switch -- otherwise these waveforms could easily be mistaken for AC.