Let me start from the beginning. And let me warn you, this isn't easy for the non-tech savy. Or without risks.
Stock means no change from original manufacture. Like stock car racing. No modifications from the original product.
Amazon removed USB host software from the Kindle Fire. You need that software to recognize mice, keyboards and flash drives. To get that software you first need command line access. To get that requires "rooting" your Kindle Fire. Be aware that voids your warranty. Google "Root Kindle Fire." After you root, you have to install a mod, I believe cyanogenmod 9 is the mod of choice for people trying to get USB hosting on their Kindle Fire. Simply google "cyanogenmod 9." You also need the latest Kernal drivers. 3.0. That should install with the latest CM9. I believe.
Once you get all three of those on (a somewhat lengthy process), you need a simple $1 OTG cable and your device should recognize keyboards. It's a lot of work for a keyboard in my opinion. And if you don't know what you're doing you can risk damaging your device. But it is possible.
Also, after you do all of that, your device won't have the same look to it. It will look more like a regular Android Tablet then what Amazon ships. In my personal opinion, I don't think it's worth doing for most people who like how their KF looks and works. But I do think that people should know what their device can do.