Question

it is told that common collector amplifier is used for impedence matching.what does it physically mean in circuits?

Answer 1

impedance matching is done for maximum power transfer between the stages. maximum power transfer can take place only when source and load impedances are equal. so output impedance is matched to the input impedance of the next stage for max power output.

Answer 2

jayantb1019 is right, and sometimes, the impedance control also used to get expected voltage level, for example, assume gate a is output and gate b is input from gate a, for gate a, the output voltage level is 5V when open, when you connect b to a, you should take the output impedance of gate a and input impedance of gate b into consideration, as the voltage of gate a output now will dependent on them and not 5V any more. (like a voltage divider, :-) )