What exactly is framework-res.apk As far as I understand it, it is a big system file that apparently controls a big part of the phone. I am interested because I wanted to do some themeing and so far I had edited SystemUI.apk, but if I ever ran into any problems (and being a beginner that was bound to happen) nothing seemed to happen other than the fact that I lost my notification bar. 

Problem is, I have heard of some people who touched framework-res.apk and they wound up not being able to even boot their phone. I decided before carelessly poking around with something that could potentially soft brick my phone, I would like to learn a bit more about it.

So what exactly is framework-res.apk? What is it used for? What does it control? Where do we visually see its use? What could potentially happen to my phone should something go amiss with framework-res? Anything else I should know before poking my nose where it shouldn't be?
Framework-res.apk basically contains the elements of the Graphical User Interface for the phone. This file is available at /system/framework/framework-res.apk. 

Poking in this file would mean changing the complete look and feel of your device. Since it is the main element of your screen, replacing it directly by pushing it through ADB would lead to soft-brick. Therefore, this apk should carefully be modified and replaced using the recovery module of the phone. 

Check this link for details on the same: Modifying the framework-res.apk (Theming the UI)