Q.What is meant by frequency response of an amplifier?

An amplifier does not amplify all frequencies equally. The frequency response is a plot of voltage gain against frequency. It is flat at a maximum value (the mid-band gain, $A_m$) over a middle range, and rolls off at both the low and high ends.

At low frequencies the coupling and bypass capacitors have a large reactance $X_C = 1/(2\pi f C)$; in series with the signal path this attenuates the input and the gain falls. At high frequencies the small junction and stray capacitances have a low reactance; shunting the signal, they again reduce the gain. In the mid-band both effects are negligible, so the gain stays almost constant. …