Clipping removes part of the positive or negative peaks of a signal or both.
Silicon diodes do not conduct until the applied voltage exceeds about 0.6 volts and only when the anode is positive with respect to the cathode.
The circuit is like a potential divider with the diode part being high resistance for voltages below 0.6 volts and low resistance above.
Fig. 1 shows the waveform into the clipper.
Fig. 2 is the output of a positive clipper and fig. 3 the output of a negative clipper.w
Fig. 4 has both peaks clipped and is often used as a LIMITER where the output must not exceed 1.2 volts.