An Elastic IP (EIP) is an IP address that you can reserve from AWS for your account. Once you've created an Elastic IP, you can assign it to any instance of your choice. Once you reserve an Elastic IP, nobody else can use that IP address. Elastic IPs are unique because they are dynamically remappable IP addresses that make it easier to manage servers and make global changes in the cloud.
For detailed information on using Elastic IPs in RightScale, see the pertinent section of the Cloud Management Dashboard User Guide.