Clone MAC address is used especially in hotels, hotspot etc when the server checks your mac address. For example, if you got your smartphone registered on the network, you can clone your smartphone's mac address to the router so that the router can connect.
Clone Mac button and you will have the following interface.
Your current mac address will be displayed in
WAN MAC Address. You can click
Clone to copy your device's mac address or click
Restore to restore the factory set mac.
Don't forget clicking
Apply and wait some seconds for it to take effect.