"Terrain.activeTerrain" what does it do?

I know this sounds like a stupid question but how does it really work? If I have multiple adjacent terrains in my game how do I know which are active? What activates a terrain?
Thanks!

Well it is supposed to return main terrain in the scene…
IN CASE, if u have made several terrains and want to find the main one of them…

http://unity3d.com/support/documentation/ScriptReference/Terrain-activeTerrain.html