Currently you have the number of sandboxes that you signed up for as a client. These are the things developers use to develop new code for the site and the idea is one developer, one sandbox. You can also pay for more at a monthly rate.
This new method allows us to tear up and tear down sandboxes at will, meaning we only pay for what we use and we can scale up and down quickly depending on the level of projects and work.
It will give either your internal team or your SI, or both, greater flexibility to do large projects while doing support or many projects at once, or accommodate some temporary resource for 2 weeks to help things along, etc etc, while keeping things cost effective.