Network Address Translation (NAT) allows private IP addresses (RFC 1918) to be translated into another IP address. An example would be devices, such as computers, that are sitting on the private LAN trying to access https:// www.cisco.com on the internet.
Because their private IP addresses are non-routable on the internet, each device would require a public IP address. Using NAT on the Cisco router or ASA, the appliance allows devices with private IP addresses to be translated to the public IP address (which is on the internet-facing port). NAT allows a network to be hidden behind a single IP address for security purposes. It is also used to conserve the public IPv4 address space.