"To restart the phone" means turning the phone off and then turning it back on again. It helps fix small technical issues and refreshes the system, just like when you reboot a computer. Full definition
In either case, you may need to also restart the phone in order to finish the activation process. (tomsguide.com)
The way to get around this is I have to restart my phone before playing it if not it will crash pretty much every time I talk to someone. (toucharcade.com)
In short, new software updates will be ready to go instantly, as opposed to other cases where users must first restart the phone in order for the changes to be applied. (nashvillechatterclass.com)