First we have to know about capacitor operation.
Reactance of a capacitor 
.
In this all are fixed values except frequency 
.
That the operation of a capacitor depends on the signal frequency.
For dc signals 
, so reactance of a capacitor to dc signals 
For ac signals reactance of a capacitor 
If the frequency is high reactance of the capacitor 
So we can conclude that capacitor can allow ac signal through it and blocks dc signals.
From the above circuit we can write the following three statements:
(1)
Capacitor 
couples the input ac signals to the base of the transistor and blocks dc signal.
(2)
Capacitor 
bypass ac signals (noise signals) to ground, which will developed at emitter terminal.
(3)
Capacitor 
couples ac output signal to load or next stage of the amplifier.
Signal magnitude at the emitter terminal:
(a) For ac signals amplitude = 0V
(b) In dc case 
I hope this helps you....................