Can someone link me to the terrain artictle about letting players in game edit terrain?

or explain how to do it.

The Roblox wiki already has an article all about this (just make sure your game complies with FilteringEnabled to prevent issues; you can just google search that).

Here's a link to a very useful article to teach you how to call these via script (scroll down and you'll see it): http://wiki.roblox.com/index.php?title=Smooth_terrain