You can create the effect of wind on your terrain by adding one or more objects with Wind Zone components. Trees within a wind zone will bend in a realistic animated fashion and the wind itself will move in pulses to create natural patterns of movement among the tree.
A wind zone object can be created directly (menu: GameObject > 3D Object > Wind Zone) or you can add the component to any suitable object already in the scene (menu: Component > Miscellaneous > Wind Zone). The inspector for the wind zone has a number of options to control its behaviour.
The Mode can be set to Directional or Spherical. In Directional mode, the wind will affect the whole terrain at once while a Spherical wind blows outwards within a sphere defined by the Radius property. Directional winds are more useful for creating natural movement of the trees while spherical winds are more suitable for special effects like explosions.
The Main property determines the overall strength of the wind but this can be given a little random variation using Turbulence. As mentioned above, the wind blows over the trees in pulses to create a more natural effect. The strength of the pulses and the time interval between them can be controlled using the Pulse Magnitude and Pulse Frequency properties.
The main use of wind is to animate trees but it can also affect particles generated by a particle system using the External Forces module. See the reference page for Particle Systems for further details.