It prevents the voltage from going beyond a certain value. A Zener diode is one example. It draws high current if the voltage one direction is over about 0.3 V and in the other direction it is dependent upon the doping level but usually 3-50 V.
[edited part. The comments below are correct. One form of the circuit puts one end of a waveform at a specific voltage. This is usually done wit a series capacitor and a shunt diode to a voltage.]
It is a circuit that shifts the DC level of a particular input. it mostly uses diodes and capacitors.... the main idea would be to charge the capacitor to a particular voltage...