What kind of wave motion is described by grass moving in the wind?

Each blade of grass is displaced from its equilibrium position according to the local velocity of the wind. This means the waves are actually periodic variations in the windspeed. This is likely due to vortices generated as the wind passes upstream obstacles, such as hills/woodland/buildings.

Here is a more realistic rendering of vortices, albeit with a slightly different geometry (the airflow passing both above and below the obstruction). This phenomenon is known as a Kármán vortex street.