uses of AUX port

AUX cable is black. AUX port is usually used to get dial-in access to the router.

For getting dial-in access to the router, you will need to connect a modem to the AUX port of the router using Cisco AUX cable. Then you will need to connect a telephone line to this modem. Once this is done, you can use any utility like Hyper Terminal to dial the number of the phone line and get console access to the router remotely.