open question argument

As I sit here, lap top in my face as i research topics for my first essay, I run into an article on Wikipedia on ethics and the "Open question argument." As some may know this argument was presented by the British philosopher G. E. Moore which sets out to illustrate the "indefinably" of the word "good" (which I assume to mean the word cannot be defined). As I read on click on links to help me understand the problem and argument a bit more and all I do is get more confused. A quote from Moore's book later helps me understand it a bit more but still I sit and ponder what exactly he wants to say. This is the quote...

"That "pleased" does not mean "having the sensation of red", or anything else whatever, does not prevent us from understanding what it does mean. It is enough for us to know that "pleased" does mean "having the sensation of pleasure", and though pleasure is absolutely indefinable, though pleasure is pleasure and nothing else whatever, yet we feel no difficulty in saying that we are pleased. The reason is, of course, that when I say "I am pleased", I do not mean that "I" am the same thing as "having pleasure". And similarly no difficulty need be found in my saying that "pleasure is good" and yet not meaning that "pleasure" is the same thing as "good", that pleasure means good, and that good means pleasure. If I were to imagine that when I said "I am pleased", I meant that I was exactly the same thing as "pleased", I should not indeed call that a naturalistic fallacy, although it would be the same fallacy as I have called naturalistic with reference to Ethics."
– G. E. Moore, PE § 12.

I can understand Moore argument a bit more than the responses by the naturalist such that, I believe Moore sees "good" as a word that can only be defined with other words such as nouns or verbs but can not actually by simplified because it already at it's simplest point and his reasoning behind it is empirical. There are no actual facts on how to define what is good but naturalist want to say one has to experience what what is good. I ask the question though, which i guess is similar to how i started writing this, how do we know what we are experiencing is "good" if we cannot even define the word?