In this circuit you can see three-phase-current. It is displayed as the three 230v AC supplies with 120° phase shift from one to another.
The three diodes will get rid of the negative voltage which is possible due to the phase shift. For example I'll call the AC supplies PSU 1, 2 and 3:
PSU 1 is at max voltage (230v) and pushes electrons through diode one. Additionaly it is "pulling" electrons from PSU 2 and 3 because they are currently at negative voltage.
This procedure repeats over and over for every three PSUs one after another.
The problem is:
If you are watching the blue-marked lamp, you can see, that the now operating DC is very "rough" because of the AC.
Solving this problem is very easy due to the properties of capacitors; the capacitor is charging at the peaks of each cycle and powers the load (in this case the green lamp) while the DCs voltage is lower than the operating standard.
Shortly: the capacitor reduces the differences made by the original AC.
I hope you were able to understand the basic function of an AC to DC converter with three-phase-current.
