Help me understand

I'm really new, so go easy.  I don't understand the concept behind the "openings".

So I don't know any of them, but try to follow with my made up example.

Let's say you want to use the caro-kann defense, and it goes 1. e4 c6 2. d4 d5, but instead of 3. Nc3, he does say, Bf4?  What happens then?  Do you have to know another opening that has 3. ...Bf4?  Or does each opening have ways to deal with moves that you're not expecting?

Does that make sense?

I think it is more helpful to think about openings being about ideas rather than particular move sequences. You, for example, with the king's gambit white puts pressure on black's e5 pawn, exerts pressure along the f file, and aims to attack the f7 square. But your opponent will have their own ideas and you may have to change your plan depending upon what happens during the game. 

Sorry, this turned out to be much longer than I originally intended. First I answer what the general point of openings is, then I move on to your last question.
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Ok so, in the starting position, the pieces (pieces are the non-pawns) aren't working together to do anything. Most of them can't even move! Luckily this is true for both players... but the point of the opening is then to organize your pieces as quickly as possible.

Now, it turns out the center is important. Imagine a piece in the center for example. They can quickly  move to any other part of the board, and can even simultaneously assist in attack and defense.

Also notice that you may be organized, but if your king is under attack nothing else really matters. Or at least, threats to your king override anything else going on.

So these are the opening principals, they are the point to the opening:

1. Claim some central space with pawns
2. Bring your knights and bishops off the back rank as quickly as possible
3. Castle to safety

When this is completed, and your queen also leaves the back rank, development is complete, and you'll be ready to start operations against the opponent.

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A few notes. Pawns are useful to control space because they're the least valuable... it means no other piece can land on a square a pawn attacks, the trade is not worth it.

Knights and bishops come off the back rank first for the same sort of reason. Everyone but the pawns are forced to run away from the attack of knights and bishops. This is also why the queen is usually brought out last. 

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Now on to your last question, do you have to know a new opening for every move.

The short answer is no. If your opponent plays an obscure (or even a new) move in an opening, then it's just some variation. However the most basic themes are the same (to control the center, develop quickly, and castle).

Also, it turns out the strategic themes, the common middlegame ideas, are highly related to the pawn structure. So new moves don't often change the main ideas.

For example in the Caro Kann pawn structure an early goal for black (soon after development is done) is to play the pawn break c5. So lets look at Bd2 vs Be3 vs Bf4 vs Bg5. Black may look at Bd2 and think, oh, this somewhat blocks white's d file, so if my move c5 opens the d file that will be more effective for me. They would look at Be3 as helping to prevent c5, Bf4 as perhaps probing the queenside, controlling b8 so after c5 openings things maybe a rook can't go to b8. They would look at Bg5 (lets pretend it pins a knight) as not having anything to do with c5 really, so as long as the knight is defended (and there are no immediate tactics) it's mostly ignore-able.

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Anyway, so new moves are made sense of though the framework your general knowledge provides. Sometimes a move will completely change what you aim for. Lets say really early the in caro white play c4 then plays c5 himself. So now what? Then your general middlegame knowledge has to find new plans. In this case it's likely white's d pawn will be a backward pawn on a half open file for example. Also d5 is a hole.

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Hope that helps... like I said, turned out to be much longer than I intended!

1. Control the center.

2. Develop minor pieces toward the center.

3. Castle.

4. Connect your rooks.