FREE: $5 Amazon Gift Code! GIN available

This may be a dumb question. But I am seriously confused about something.-- ok:
Amazon.Com is the website. Ok and I understand If I get a (let's say) $10 gift card/code I can go on amazon.com and redeem the code and it will add the $10 to my "amazon wallet". Which I can use to buy items from amazon.com - right, simple enough.. Ok--
so- my question! I have a kindle fire HD-- Which runs off/uses the "amazon App Store". -
If I were to get a (Again lets say) $10 amazon gift card/code, and add it to the "amazon wallet" online at amazon.com-- I can --(or can't) use THAT $10 to download apps & books on my kindle THROUGH the "amazon App Store"??

Or do I have to put the code directly INTO my kindle in the Amazon App Store- and NOT put it on my "amazon wallet" online at the Amazon website?

I'm so confused! Because, I already have "$$" in my "amazon wallet" on Amazon.com --- However-- if I go to my "amazon App Store" on my kindle-- it DOESNT say That i have any funds in my "amazon App Store". ...

So are the two completely separate?? Or am I seriously not doing something right? Or is there a DIFFERENT AMAZON gift code/card that has to be bought if I wanted to download from the App Store than there is if I wanted to put the money in my "amazon wallet" to buy items from amazon.com???
** ANYONE who has any knowledge at all, on what I'm "trying" to explain here PLEASE MESSAGE ME!!! lol! i can't tell you how annoying this is that I just can not figure it out. Lol!!!
My iPhone is easy! If I want to download apps from the App Store on my iphone- I get a ITunes card. And if I want to buy something from the APPLE store/site I just get a apple gift card. But amazon has me all confused!!!
Help???