It is very common for web applications to be deployed to various locations. The most common use case is to have a staging and production environment. Before deploying new code to all of your users, you deploy it to a staging server and test it there.
Environments and sites come together in the form of a grid. You need to provide Rainforest with one URL for each intersecting cell on the matrix created by sites and environments that can be accessible to our testers. If your environment is behind a firewall, check out the list of IPs to whitelist for our testers to gain access.