What is a NAT Instance? (54 of 285 technotes for 2015)

Instances that you launch into a private subnet in a virtual private cloud (VPC) can’t communicate with the Internet. 

You can optionally use a network address translation (NAT) instance in a public subnet in your VPC to enable instances in the private subnet to initiate outbound traffic to the Internet, but prevent the instances from receiving inbound traffic initiated by someone on the Internet.

References:

http://amzn.to/1aW2pVE