Diode clamp circuit is an interesting circuit and also clever circuit design trick that one must know. The circuit limits the range of output voltage using a diode. For example, the 1N4148 switching diode can be used to clamp the output voltage to 5.6V as illustrated in the picture below.
As you can see in the circuit diagram, even if the input voltage is 10V, the output voltage is 5.6V. Thus a fixed output voltage is obtained. So from negative voltage(but now lower than -75V breakdown voltage of 1N4148) all the way to 0V to higher DC input voltage the output will not exceed 5.6V. Thus the output voltage is said to be clamped.
The idea and application of such circuit is to protect the circuit. Such circuit tricks are also used in various types of power supplies, especially at the output. See diode in buck and boost converter example..
