Analog-to-analog conversion, or analog modulation, is the representation of analog information by an analog signal.
One may ask why we need to modulate an analog signal; it is already analog.
Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us. An example is radio.
The government assigns a narrow bandwidth to each radio station.
The analog signal produced by each station is a low-pass signal, all in the same range.
To be able to listen to different stations, the low-pass signals need to be shifted, each to a different range.