DC to Dc converter module, control the DC output voltage from given DC input voltage. Idea is to generate a different voltage BUS from a given DC bus. This is done by switching on/OFF switch at moderate to high frequency. The average value of this switched waveform is controlled by controlling the ON time. This is right most block in bottom.
Second block in Top is block to generate DC voltage from input AC mains. It shows two option, full diode bridge or SCR based bridge.
Hope this helps.