To power cycle a device is to turn it off and on again. It is one of the most useful troubleshooting steps you can perform when a device is not working as it should. Power cycling a device is similar to rebooting it, but typically includes a short wait of 5 to 10 seconds where the device is completely powered down before turning it back on.
The point of power cycling a malfunctioning device is to clear the contents of its volatile memory, including its RAM. Corrupt or unexpected data in a device's RAM often causes it to crash or fail to perform tasks correctly, so clearing its contents can often resolve unexpected behavior. Power cycling a networked device also resets all network connections to other devices in case of any addressing conflicts. For example, power cycling a modem resets its memory, which clears its caches and temporary data; it also resets its network connection to the ISP, which grants it a new IP address and may help resolve connectivity problems.
While power cycling can help resolve problems with most electronic devices, you should be careful when power cycling a computer suddenly without good reason. Since it clears the contents of the system's RAM, any unsaved work will be lost. Many computers perform extra hardware checks after an unexpected shutdown, so it may take longer to reboot. If a computer unexpectedly shuts down while writing a file to a disk, that file may be corrupted and unreadable when the computer restarts.