Scripting is the automation of certain functions.
Some people claim that you can basically "script" legally using UO Assist, but when most people say "scripting" what they refer to is the use of unapproved, "illegal" 3rd party programs that allow the automation of an alarming number of functions to an alarming degree.
I've heard stories, some credible some not, of some incredible things that can be done with scripting.
But one thing scripting can definitely do is take healing functions totally out of your hands as a player. And that poses obvious advantages. (The same program that can script mine can also script heal, script run, script tame, etc.)
Healing scripts of all sorts are extremely common in PvP and have been for years and years.
Some cheaters in PvP would be able to do well even without cheating, and some can only do well because of the cheating.
-Galen's player