You can get this kind of explanation either from a specialized text on the opening, or from a stronger player who knows the opening you're interested in.
But it's also a good exercise to try to understand the move given by theory. If you have another move in mind, you can look at the database to see what strong players play against your intended move. If what you see doesn't convince you, then you may as well play your move. If you're convinced, then you've answered the "what if..." question.
Let me give an example :
1.e4 Nf6 2.e5 - here theory states 2...Nd5 is best, but I wonder about 2...Ne4
I look at the database, and look for a game played by a strong player as white here. I find Moroz - Vavra, 1998. The next moves are : 3.d3 Nc5 4.d4 Ne6 5.f4 - this looks already awkward for Black, as white is building a super-strong center and kicks our Knight in the process, that can't be good...
Now it doesn't answer the question why 2...Nd5 is good in the 1st place. Well, usually good opening moves follow some opening guidelines, such as "fight for the center, develop your pieces, defend against threats, prevent your opponent from doing this, etc." Usually, you can expect the theory move to follow those principles. If it doesn't, try the "what if...?" question, and you may come up with an explanation, or a good personal idea
When I get this new interest on chess I have being dealing with the opening question in several different ways, and none of them seams to be satisfying enougth.
My first aproach, and still the deepest aproach I have made, is to NO play openings by memory, but to study each position as it is and then make the best move I can find. This gives me 2 problems: 1. time consuming, I have a high rate of blitz games losses over this, basically any 10 minutes game that goes over 25 moves I loss on time. 2. My analisys of the position are not good enougth. This is, in some of the games the oponent is playing what some grand player did, and I find myself up to 10 or maybe more moves playing agains Fisher, Alekin, Ruy Lopez... and the so. Their analys are way better than mine and they beat me on the board.
This second point comes clear when I consider that my standad chess score is 200 points less than my 960 score. This is, when the oponent is away from the theory I perform 200 points better.
So as my score goes up it becomes clear the necesity to be aware of the opening theory. I get 2 tools for helping me on this. One is in this website, where I can see the moves in terms of stadistics, what move is more popular and how many of the games ended on victory for one of the players or draws. The second tool I downloaded is for free in another website. It basically have the openings on a database; So the program shows me the opening, I have to memorice it and then play it, some sort of "Simon game" but in chess instead of colors. I trained for about an hour whit that tool and get the Alekin memoriced up to move 6 and some of the variations up to move 10.
My problem with this 2 tools is simple. I don't understand what I am playing. I know it is good, even better than my game, but I don't know why.
So what I need is something that explains to me the reasons behind the moves, the explanation why this move is better than the other, and the particularities of say position.
Is there something out there like that? preferable, if is on computer, still I don't mind books or other resourses.
I appreciate advice on this matter of mine, and probably of other players arround.