There are three basic kinds of mechanical waves: Transverse waves, longitudinal waves, and surface waves. Surface waves propagate along an interface between differing media.
One way to describe wind blowing across tall grass is as a surface wave. The grass is held in place by its roots. The tops and sides of the grass present a surface to the wind similar to water, which is held in place by gravity and surface tension. Wind blowing across the surface of water creates circular motion of water molecules that appears as surface waves. Likewise, wind blowing across grassland may create a modified circular motion of the grass stalks.
The grass rises and falls in sinusoidal waves subject to the restoring force of the grass stalks and the density of the grass.
In the case of waves through grass, most of the energy probably is carried by air, although some may be transferred from grass stalk to stalk.
Another possibility may be to describe this phenomenon as an unstable bedform. The shapes are wavelike, like ripples in sand, but grass doesn't hold its shape as long as sand because of the restoring force of the grass blade. The wave shapes are caused by variations in windspeed.