Frequency response is the measure of any system's response to frequency, but is usually used in connection with electronic amplifiers and similar systems, particularly in relation to audio signals. For example, a high fidelity amplifier may be said to have a frequency response of 20Hz - 20,000Hz ±1dB, which tells you that the system responds equally to all frequencies within that range and within the limits quoted. As such it is not a measure that is very useful in terms of the quality of reproduction, only that it fulfils the basic requirements needed for it.
A related term is bandwidth, but that has both a more tightly defined meaning, and a broader application.
See also
de:Frequenzgang
