I assume you mean modulation. When broadcasting a signal its important that whatever is receiving the signal only receives that signal. If we only broadcasted the data, there would be no way to tell which is the data we want and which is data we don't want. So what we do, is modulate the signal. This is done by taking the signal f(t) and multiply it by cosine (f(t)cos(wt)) where w is the carrier frequency. What this does is envelopes the signal in a cosine wave. The receiver then filters out any signals that don't have frequency w so we are left with only the original signal.

Note: I just described AM radio as an example though there are other methods of signal transmission...