What does "to root a phone" mean?

Other answers are already correct, but just for a bit of nerd reference. In all Unix-based systems, there is a "root" user who has access to any and all functions available by the operating system. This is a very powerful (and dangerous) privilege because once you are root, you can do anything, and much of what you can do can completely destroy the system.

This is why when you administer a server, you usually don't do it as root so that you can take advantage of the operating system saying "umm... you don't want to do THAT!" when you accidentally ask it to wipe clean the entire hard drive at 3am.

To 'root' your phone means to gain administrative rights on the file system of your phone (in linux, root is the username of the master admin). With root access, you can install and uninstall anything you want on the phone.

Most phones come with limited access regarding what you can and can't do on it. This isn't necessarily a bad thing, because it can keep users from accidentally breaking something they shouldn't mess with on the phone, especially in regards to the operating system. However, many manufacturers limit your rights to things that aren't really so mission critical, too, and rooting the phone gets around this.

If you have a few unnecessary applications (bloatware) pre-installed on your phone that you cannot uninstall, rooting will give you this ability. It will also allow you to upgrade to newer versions of Android before your phone's manufacturer and/or cell provider make the updates available to you. Be warned, though, that rooting may void your warranty.

For more info on what rooting enables you to do with your phone, check out this question.

This is a bit of an amendment to everything. Essencially Rooting is Jailbreaking. You are a user, you don't have administrator access to your own phone. Rooting is the process of getting access to the "root" user. Once you have root, there are some fun things you can do.

• Remove "system" apps like Sprint's Nascar or NFL apps.
• Use tools like Open Tethering to do wifi tethering (for free)
• Underclock or Overclock your processor (underclocking with SetCPU helps keep battery usage up while not really affecting performance when your phone is in idle/etc.)
• Install a custom recovery tool.
  • Nandroid Backup -- A complete dump of your android's internal memory to the SD card which allows you to do a very clean backup and literally roll your android back to that point when needed. Also allows you to swap operating systems in minutes by having 2 backups, 1 of each.

Recently rooting is getting more efficient and with 1-click rooting coming up, it should be a breeze for most people, just set up and let it run for a half an hr.

The only risk is bricking your phone (screwing up some critical software thus making it impossible to start it up or install a fix).

To gain root (administrator) access to a phone.

'root' is the user with UID 0, which is the equivalent of a full 'System Administration Account'.

It means you have the permission to do anything. Anything includes opening protected ports (for example for WiFi-tethering), update and upgrade the OS and all software, replace the operating system with your own software, full access to every file on the machine, and the ability to delete any file, including the entire operating system.

In short, it means to get administrator access to the phone. To do things that a normal (non-administrator) user of the phone cannot do.

When you "root" your phone you are unlocking the administrative privileges of the phone which by default are locked by the manufacturer. It's similar to "Jailbreaking" in the iphone world.

Rooting your device allows you to do things like overclock or underclock your CPU, delete those preloaded apps that your phone wont delete like Amazon MP3 or Corp Calendar, install certain apps that require root access, load custom ROMs...

The manufacturers don't want you to have access to these options for a number of reasons including but not limited to:

• They want to control and ensure a specific user experience
• Certain carriers require that certain functions are locked out because they want you to pay for them or because they are incompatible with their network
• If you don't know what you are doing you can "brick" (render useless) your phone or at least cause enough problems where you'll be calling tech support demanding to know why your phone doesn't work.