The material through which a mechanical wave travels is

The medium is the main factor that differentiates a mechanical wave from an electromagnetic wave, since the first can not propagate without its existence, while the second can propagate regardless of whether the medium exists or not.  

In addition, it is the medium that will define, the propagation speed of the wave, according to its specific physical characteristics.

think about the oscillation of the toy shown in the video. when the toy is at the highest point of its motion, its velocity is zero and its acceleration is negative.