How to understand an opening?

I'm confused about how a chess player understands an opening. For example, take the Nimzo-Indian. How do you understand the plans for each variation and what to do? Is it all explained in a book or do you have to sift through master-games in that variation or do you come up with a plan by yourself? It seems like I can memorize the theory, but I can't understand the plans behind an opening and what I'm supposed to do. How do I understand the plans behind an opening? Thank you. 

Hi in general I suggest to get an understanding:

a) Have a more experienced player of the opening explain it to you. (Similar to a good book I guess)

b) Practice playing the opening. If you think a move that isnt theory is good, try it out and see how you go. (often this has taught me why the other moves are played).

c) Go through master games that have played the opening. See the way they play the different variations.

Over time you will understand why the moves are played the way they are, due to the countless variations you can spend a lifetime learning an opening well. The more experience you have with it, the more and more you will understand the ideas.

goodluck!

Maybe this explanation will help you.

No matter what opening you play.  The goal is to create an imbalance.  Maybe that imbalance will be you have more space on the queenside or if you want to exchange your bishops for his knights. 

Now once you have reached a position that has an imbalance you must try to make that imbalance show you how you should play your position for example.

You trade off your dark square bishop for his dark square bishop. This leaves you with both light-squared bishops.  Now your plan should be to put your pawns on the dark squares. This will make your bishop good but also force him to have his pawns on light squares hence making his bishop a bad bishop.

In the Nimzo-Indian there are times where Black trades off his dark squared bishop for the knight on c3 to give white doubled pawns if he can and a weakened queenside.  Although normally blacks pawns are on d7 and e6 because of this opening. Black will usually play for d6 and e5 putting his pawns on the dark squares so that his bishop on c7 will be able to have free reign of the light squares.