Rooting (Android) and Jailbreaking (iPhone), are hacks and/or custom operating files that unlock the phone from all of the restrictions it manufacturer and/or carrier have placed on them. i.e. If you wireless tether officially the carrier charges you a separate fee above and beyond your regular data plan (which is BS). If you jailbreak or root and install a tethering app, you can tether without the carrier knowing you're doing it.
There aren't instructions in the manual (especially since it may void your warranty), however, the instructions are readily available online, as are the programs for doing it. Most of them, you run a program on your computer, do a couple of specific keypresses on the phone, and voila, rooted/jailbroken.