It allows you to flash custom ROMs, radios, recovery images, etc. A bootloader just refers to what code is executed when the phone starts. Flashing a ROM replaces the factory code.
Don't trade your phone if you're doing that $0 down, $20 a month for your phone bull. You will be breaking the law.
No. There's no way to "hack an app" and get a free subscription to a service like Hulu and Netflix.
You can change how apps look, both icons and general UI, and a plethora of other things. But it depends on a couple of factors. 1. Your device, and its capabilities. 2. Developers, and what sort of customized things have been developed for your device. (Of course, you can become a dev too...but I'm guessing you aren't there quite yet)
Hit YouTube and watch some videos about rooting your Kindle. Once you're familiar, try it.
S4 is trash. Gabage. Get the M8. The G2 is a nice phone, but the M8 is better. Barring some manufacturer errors that might come to light later, the M8 is the superior device.
With time, you'll learn
@Superb is to be ignored. The millionaire Apple employee/startup founder/LG G2 fanboy/Loan & Tax expert has a rudimentary understanding of most things.
You can already set .gif images as your lock screen image, but i suppose you want it to be animated too, right? That'd be terrible, considering .gif compression isn't all that great and would be taxing to keep in the memory. We already have live wallpapers. If its that important, just create a live wallpaper. Its not that difficult.