Undulate means to move in a wave-like pattern. If a sound increases and decreases in pitch or volume like waves, you can say the sound is undulating. When searching for the lost boy, the rescuers' cries undulated through the forest.
The verb undulate comes from the Latin word undula, which means "wavelet" (unda means "wave"). So any action shown with the verb undulate has that waving sense to it. If something moves in a wavy, pattern, for example, it is said to undulate. The wind might cause prairie grass to undulate, or music might cause hips to undulate.