What does NAT stand for?
NAT stands for Network Address Translation, and describes the process whereby TCP/IP traffic is remapped from one set of IP-address/port numbers to another. When used on outgoing connections from a LAN to the internet, this accomplishes several goals: 1) Allows exposing fewer or no IP addresses on the open internet 2) provides a casual measure of security and privacy by hiding internal IP addresses 3) uses as few as one (possibly scarce) official IP address for an entire internal network. Note that (2) is not real security, and is no substitute for a full-blown firewall with a well-thought-out set of firewall rules, done by a knowledgable security architect .
