Modulation is a process of modifying an input signal in its attributes such as frequency, phase and amplitude. The resulting signal is called modulated signal.
Another signal, called modulating signal, is used to module the input signal.
Modulation is used for multiplexing or putting together thousands of signals like radio channels, together. Modulation is a sort of encoding technique which is done on analog or digital data signals in telecommunications. The modulated signals are transmitted in air or through a cable.
The received signals on the other side of air in the atmosphere or a long wire, are passed through demodulating process. Then the data signals are recovered.
Modulation is done to improve the quality of signal when there is noise in air/cable. It is done to reduce errors in data transmission.