To vary a
carrier frequency in
relationship to a
signal. This is really only
useful if you can
reverse this
procedure or
demodulate. This is the
basis of a
modem.
Some famous uses of this are
FM or
frequency modulation and
AM or
amplitude modulation on your
car stereo.
One of the goals of modulating are to put many different signals on different carrier frequencies so that they can all use the same
medium of
delivery. These mediums can be
wires (
cable TV),
air (radio and
WLAN), etc. Other common things that use this are
Infrared Remote Controls that use
light in the Infrared as the carrier frequency.