What is mean by limitter diode?

Answer 1

Diodes theoretically conduct in one direction but not the other. In practice, diodes require a certain amount of voltage too make them conduct. We call this "forward bias". For silicon, it's 0.6 volts.

If we connect two silicon diodes in parallel, one in each direction, across a 2 wire port (let's say it's carrying an audio signal), as long as the signal says below the forward bias voltage, nothing happens. When the signal exceeds 0.6 volts (positive and/or negative), the diode conducts, and the portion of the signal exceeding 0.6 volts is shunted, which limits the signal to 0.6 volts.

For other voltages, we can use Germanium (0.2 volts), zener diodes, and such.