The second part of the Wikipedia definition explains that Estate Planning "attempts to eliminate uncertainties over the administration of a probate . . ."
Probate? This sounds complicated. Will it hurt?? Well, only a little bit. (Just kidding . . . sort of).
Seriously though. When you die, in most circumstances a special court, called a Probate court, oversees all transfers of your stuff from you to your friends and family. This "overseeing" process is called Probate. The Probate process makes sure that all your debts are paid and that everyone gets the stuff they are supposed to get.
This process is made a lot easier if you have a properly drafted Will. But if you don't have a Will or if your Will is confusing or unclear, the Probate Court may have to hold hearings, interview heirs, and perhaps even hold a trial just to figure out where you want all your stuff to end up.
In essence, proper estate planning makes going through the court "smooth sailing" so that your stuff is not tied up in this Probate process for months or even years.
To summarize thus far: You first need to decide who gets all your stuff when you die and then make sure you clearly indicate who gets this stuff (i.e. Will) so you don't get stuck in probate. So far, so good? Well, the fun is just beginning...