Any resources to understand Clamper circuit?
If the source (represented by the circle with the diagram on the left in your schematic) tries to output more than +/- 20 V plus one diode drop, current will start to flow through the "clamper". When this happens, the "excess voltage" will drop across R1 and the output will only see ~20.7 V.

For example, say the source tries to generate +30 V. Then current will flow through D2. This current will also flow through R1, creating a voltage drop. The net result will be an output voltage of about 20.7 V (could be 20.2 V to 21 V, depending on the exact type of diode, value of R1, etc).

Aside: the elements drawn as batteries in your schematic should not generally literally be batteries. Something like Zener diodes is generally much more appropriate. If you did actually build this with batteries, the internal resistance of the battery would affect the clamp voltage, and I didn't account for this in my example above.