As a wave moves through medium particles are displaced and

When a wave moves through a medium, the particles of the medium are displaced from their equilibrium positions temporarily. The type and extent of this displacement depend on the nature of the wave.

In the case of a transverse wave, such as a wave on a string or light wave, the displacement of the medium's particles is perpendicular to the direction of the wave's travel. Imagine a string held tight between two points; when you flick one end of the string, a wave travels along it, and the particles in the string move up and down as the wave passes, returning to their original position afterward.

In the case of a longitudinal wave, such as a sound wave in air, the displacement of the medium's particles is parallel to the direction of the wave's travel. The particles oscillate back and forth in the same direction that the wave is moving. When a sound wave travels through air, areas of compression and rarefaction travel through the medium. Compression is where particles are pushed closer together, and rarefaction is where they are spread further apart.