AM stands for “Amplitude Modulation.” In AM modulation, the amplitude (or strength) of a radio wave is used to carry the information, such as speech or music. The signal is modulated, or changed, in such a way that the amplitude of the carrier wave (the wave that is being used to transmit the signal) is made to vary in accordance with the amplitude of the modulating signal (the signal that is carrying the information).
In an AM signal, the carrier wave is a sine wave of constant amplitude and frequency. The modulating signal, which contains the information, is also a sine wave, but its amplitude is varied in order to transmit the information. As a result, the amplitude of the carrier wave will also vary in the same way.
The advantage of AM is that it is relatively simple to generate and detect, and it is also relatively resistant to noise and interference. However, it is not as efficient as other forms of modulation, such as FM, and it also suffers from a phenomenon called “capture effect” where a stronger signal can overpower a weaker one.