Refer to figure.
When we want to send information via radio waves, we have to modulate the carrier wave somehow, this is the process of adding information to it. There are a few ways to do this. In the early days of radio, it was done by sending pulse of the carrier wave as morse code, pulse modulation. These days we can modulate the amplitude of the carrier wave, to carry our information, or we can modulate the frequency slightly, to carry information also.

Amplitude modulation is easier to do and far easier to represent visually, as shown above. The peaks and troughs of our carrier wave are modified to create our information wave over the top of each peak, and the same information wave under each trough. The wave over the peaks is one sideband, and the wave under the troughs is the other sideband. These signals are carried along by the carrier wave, and then decoded in a receiver unit back to their original state, giving voice messages in the case of VHF.

If a transmitter was faulty, it may not be able to encode the voice message information onto the carrier wave, and so may be able to send the carrier wave only, with no modulation. This would sound like clicks, or crackles down the radio, similar to what can be heard when a pilot presses the Push To Talk (PTT) button but says nothing. This carrier wave can be used to send signals in morse code if necessary, effectively going back to the days of pulse modulation.