Rooting and Jailbreaking refers to the same thing. The term Jailbreaking comes from Apple's iPhone community, the preferred term in Android is rooting.
Rooting/Jailbreaking refers to enabling the administrator/superuser/root/user-id-0 user on the phone. In unrooted device, the superuser account is disabled for security purpose. The superuser possess full privilege over the system, including deleting or modifying critical system files.
More about rooting: What does "to root a phone" mean?
