Modulation is a technique in which message signal is transmitted to the receiver with the help of carrier signal. Here in modulation, we combine both carrier signal and message signal. You may get the doubt that what is the need of modulation. Just imagine that you have a paper which contains the message and you would like to send it to your friend standing 40 feet from your place. You can't just through the paper to your friend because paper will not travel that much distance but if you take small stone and cover the paper with it and through it to your friend, it will definitely reach the target. In the same way, we need a carrier signal to transmit our message. Sometimes, message signal is also called as modulating signal. The exact definition of modulation is given below:
"Modulation is a process of message signal and modulating is varied according to the carrier signal for transmission purpose. The message signal can varied in accordance to the carrier signal that is in terms of angular or amplitude. So we are modulating the signal."