To reboot means to turn something off and then turn it on again, typically a computer. It can also be used to refer to an updating of a media franchise, such as the 2009 "Star Trek" reboot.
The word reboot originated from the concept of booting up a computer, which in turn came from the term bootstrapping. Bootstrap refers to a process which, once started, is self-sustaining and requires no external input, such as the automated start-up of software when a computer is turned on. To reboot, therefore, is to turn a computer off and immediately turn it on again to start from scratch.