i try to learn chess openings and i have a question

it's about the slav defense or actually more about a counterplay against it i guess.
it's 1.d4 d5 2. c4 c6 3. nc3 nf3 4. dxc4 5. a4

why the a4? i don't get it, what is the reasoning behind that move? is it only to prevent b7->b5 so black can't support it's c4 pawn, which would be a solidified structure deep in enemy territory, what is my territory ... wow, i don't think i would like that? ...which come to think of it that does make a lot of sense, but since i'm stupid and have no idea about chess, i thought better ask the experts. is that the idea behind it or is there more to it?

oh, i just saw... like at first i thought...what gives, if they go b5, i can still go a4, but of course that's nonsense, since they can take me on that square, wherew as when i go a4 first and the go b5, i can take b5and got the A file open for my rook. is that the reasoning behind it? please help me!!!!

I’m no expert, but you’re right about it stopping 5…b5. White is playing the Queen’s Gambit Accepted btw. You could probably find more info on it if you checked out that opening.

Actually, after looking at the move order and position of the pawns, I’m not sure if it could be QGA transposed or still fall under Slav, but you can find similar ideas under QGA.

As stated a4 will secure that white recovers the pawn because b5 can't really be played after that. 

Now, it is admirable for you to be curious about some positions, but to be fair, up to this position is a maximal amount of moves you need to know in that rating range. You will rarely get more theory than that, and probably most of the time, you will not get that many book moves.

It is of course good to choose a certain variation as white and as black, but don't overdo on opening study. Most important are opening principles at that rating range when it comes to opening stage of the game.

Make sure to not blunder material without a reason, as that will mostly drag you down at this point.

You are confused because OP didn't write the right moves in this position. The position on the board is actually:

1.d4 d5 2. c4 c6 3. Nc3 Nf6 4. Nf3 dxc4 5. a4

It's not QGA, it is a Slav Defense.

so, just that i get that right, there's really not much or anything more behind it (not saying that it wasn't enough on its own)? it's "just" a direct defensive move? it's not a part of a "bigger plan" ?

 Nklristic, yes, I added an edit after looking at it again. Thanks for the correction though.

Justiziabelle, yes, that’s it.

omg, you are right. so stupid, how can anyone help me if i can't even get the positions right. stupid justiziabelle, i really think you should punish me. maybe you can totally destroy me in a chess match? yes, i think that would be appropriate.

Yes, and the point of 4. ...dxc4 by black is not to keep the pawn, but to occupy white to play a4 and then e3 to get the pawn back, so he can get that bishop on f5, outside of pawn chain. 

Of course, white can do other stuff except a4, play something else, sacrifice the pawn for development and try to play actively. But a4 is certainly the main move.

Mistakes happen on every level, don't worry.

i really like that, that's great ... and a little bit sad. i should have come up with that myself, but here i am relying on knowledge from people who are probably already dead for like three centuries when they didn't even know how to mix a white russian.

sorry, that sounds disrespecful. i am really grateful for all the help and advice and background knowledge you all provide, it's just that sometimes i think " how the F didn't you think of that yourself?"

Nkrilistic, thanks for the pointers on the Slav. I don’t play it, but it’s good to know.

Chess is easy to learn how to play, but it is not easy to come up with things like these yourself. Sure, this seems straightforward, but most of the people who know a bit about certain openings didn't really figure it out themselves when they were sub 1 000 rated. They either listened to some video, or read a book, or a stronger player told them the idea, or something like that. 

In general, novice players don't know openings too much, and they don't need to. I don't need to know a lot more than this actually even though I play this as black. 

You're welcome.

yeah, that's so true (and also that three paragraphs text i wrote earlier but then deleted because  what gives).