There are three waves shown in this figure. The one at the top is the unmodulated carrier wave. There are 33 and one third cycles of a sine wave. The amplitude is constant. The second wave shows the audio wave, two cycles of a sine wave. The third wave shows the carrier wave amplitude modulated by the audio wave. The wave starts out at the left at the same amplitude as the carrier wave. The audio wave is zero at this point. The RF wave is increasing in amplitude with every cycle. The positive peaks are getting higher and the negative peaks are getting more negative. This wave reaches its maximum amplitude where the audio wave reaches its maximum. As the audio wave starts down the RF wave decreases in amplitude. Where the audio wave is at its negative peak the RF wave is zero. When the audio wave returns to zero the RF wave returns to its unmodulated amplitude.