This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
The frequency response of a device is the range of frequencies over
which that device can perform in some fashion. The action is specific
to the device in question. For example, the frequency response of the
human ear is around 20Hz-20kHz, which is the range of frequencies which
can be resolved by the eardrum. The frequency response of an amplifier
may be 50Hz-40kHz, and that of a certain speaker may be 120Hz-17kHz.
In the car audio world, frequency responses should usually be given
with a power ratio range as well, such as (in the case of the speaker)
120Hz-17kHz +/-3dB. What this means is that given an input signal
anywhere from 120Hz to 17kHz, the output signal is guaranteed to be
within an "envelope" that is 6dB tall. Typically the extreme ends of
the frequency range are the hardest to reproduce, so in this example,
the 120Hz and 17kHz points may be referred to as the "-3dB points" of
the amplifier. When no dB range is given with a frequency response
specification, it can sometimes be assumed to be +/-3dB.