What is dual power supply in electronics?

AC is supplied to the input of DC power supply. One way to get to DC is to full wave rectify the AC sine wave. Basically you flip the negative side of the AC waveform to the positive side. You are left will all positive voltages varying from the peak voltage to zero and then back to peak. To make DC you need to smooth out the valleys and make them as close to the peak voltage as possible. You do this with various types of filters that usually involve a large capacitor. Once filtered you can use various circuits to either create a fixed DC output voltage or a variable output.