The DC-DC module is realized with the idea of switching power supply. DC-DC has two kinds of buck and boost, here only say buck, for example, you give DC-DC input 10V, DC-DC has an oscillator and chopper module, for example, allow 10V to pass in one time period, and do not allow 10V to pass in another time period (equal to 0v).
At the output end, there is a capacitor for filtering, as long as the capacitor is large enough, the result is equal to the middle of the pulse waveform calculus, and the output of a 5V DC waveform.