The term power cycle, or sometimes called power cycling, refers to the act of turning off a computer and turning it back on immediately. This term could be used to describe this action for any electronic device as well, not just computers.
Power cycling a computer is sometimes done to fix issues with the computer or a software program that is experiencing errors when running or other issues. Power cycling the computer can sometimes help to clear up the errors or issues and get everything to run correctly again. When talking to a computer help desk person, they will often times recommend that a user restarts their computer, which is basically the same as power cycling the computer, as a first try at fixing computer issues.