Frequency is the number of sound waves that pass a fixed receptor, your ear for example, in an established amount of time. This may also be called a cycle.
A hi-fidelity amplifier usually has a frequency response of 20 Hz to 20,000 Hz within approximately one dB. The human ear can normally detect audio frequencies encompassed by that specific range with a dB being the loudness or amplitude. The system should be able to amplify all the frequencies in that range.
The specific numbers indicated by the frequency response curve are not as important as the variation of response from frequency to frequency (e.g. high frequency to low frequency and vice versa). Acceptable frequency response, as plotted via the frequency response curve, doesn’t mean accurate sound reproduction, only that the tested component meets basic frequency response requirements. This response indicates to the tester that the component will accept an input signal and generate a response. The frequency response curve will not, however, define the actual audio quality.
you can visit wisegeek.com for more info