Warranty nullification and bricking risks are some of the harms that a rooted phone will face.
It means that the phone is switched off.
Yes
you need to charge it
Probably
Yes
They are out of service
prohibited
He wants to plow you
Negative numbers cannot be rooted. As it can be seen on a calculator when -25 is rooted is says error. Only positive numbers can be rooted. If it resulted in a negative try going over the problem.
Rooting is a term used in Android mobile OS hacking, it provides all the advantages of Jailbreaking (if you're familiar with that) in that you can remove the crap software that the carrier has on by default, make changes to the underlying system code, and even install custom "roms" which can change themes and provide boosts in speed and reliably of the phone. The best website for learning more on rooting your Android powered phone is xda-developers.com
It means that the network is down.