Smoothing Capacitors are ByPassCapacitors on crack. They are generally very large value elecrolytic capacitors. They take rectified voltage, and smooth it out. The capacitor gets charged on the rising voltage of the rectified output, and when the voltage starts to drop the capacitor discharges, ensuring that there isn't a big voltage drop.
Note that they do not smooth the voltage 100%. There is always some drop in voltage. To get a 100% smooth voltage, you would need to use a VoltageRegulator.