I've seen tons of free-ware terrain meshes. What exactly does a terrrain mesh do for FS9??
To my uneducated eyes it looks as though it helps the scenery by making it more 3 dimensionally accurate. Is this correct?
Exactly, it makes the scenery below more believable, it does look better, although if no one told you it was there you wouldn't know it but would enjoy it.
You can get some of the better mesh here, some of it free.
Add it one at a time and back up your files in case of problems.