"Root: Rooting means you have root access to your device—that is, it can run the sudo command, and has enhanced privileges allowing it to run apps like Wireless Tether or SetCPU. You can root either by installing the Superuser application or by flashing a custom ROM that includes root access"

SOURCE: http://lifehacker.com/5789397/the-always-up-to-date-guide-to-rooting-any-android-phone