Hard resetting a phone refers to the process of restoring a phone to the state it was when it left the manufacturer.
This articles explains the process:
How to hard reset Asus ZenFone C.
You should begin by switching off your phone to stop any power flow in the handset.
Press and hold simultaneously the up volume button and the power button for a few seconds.
When you see the reset menu in the phone’s screen, release the held keys.
You can toggle through the various options by using the up or down volume keys. But since you need to hard reset your phone press the volume down button and select the “Factory Reset” option. Once selected, use the power button to confirm the selection.
Wait for the phone to do its reset procedure until you see the initial screen display that appears in a new phone. If the procedure is followed correctly, you successively reset your handset.