Pantheon provides three environments for each site: development ("dev"), testing ("test"), and live.
The development environment is where you will upload/install new modules and apply updates available on your dashboard. The test environment is a good place to experiment and test the new code against your latest content. The live environment is what drives your live website.
You can use Pantheon's "Database / Files" tab in order clone the database and/or files between environments as needed.