In this section of Waves or more specifically mechanical waves we focus on the waves which need medium to travel. The waves which need medium to travel are called mechanical waves. In case of an earthquake, the disturbance travels in the form of seismic waves.
Wherever there occurs a disturbance and that disturbance moves or propagates, the moving disturbance is the wave and this carries energy. For example, the electromagnetic wave from the Sun warms us and seismic waves can destroy buildings.
The oscillating motion has a direct relationship with mechanical waves. The simple harmonic motion in Mechanics has a direct relationship to the understanding of mechanical waves.
The information of waves lead to the understanding of atomic structure. You are supposed to understand waves before diving into atomic physics. First learn oscillaitons, then learn waves and learn atomic physics and voila!