Modulation simply means changing or controlling something. Since we are dealing with communication here we mean, changing or modulating signals. But why signals need to be changed or modulated?
It’s because, the message signal for example consider human voice which around the frequency of 20Hz to 20kHz, which is very low to transmit from one to place another. Moreover, such low-frequency signal will suffer lose along its path. Hence it needs to be changed or Modulated in such a way that it is received at the desired end with minimal loss.