The Staging Server is a the testing and developement server that is open to everyone who plays the game.
To login to the server you need to got the alternate url of: http://www.tsotesting.com/en/play
You will notice that it looks very similar to the regular login page and indeed it will feel very familiar. The difference comes once you log into the game.
Now you get the latest version of the game in an isolated environment where you can have access to unlimited resources and the chance to test out new functions and systems before they go live on the real servers.
Note: every so often the content is wiped and reset to allow testing of new content and systems so don't go building a colony that you care for.
Every so often the developers will ask everyone to start playing on the Staging server to test specific new elements.
These can range from simple stress testing (hitting the server with as many people as possible) to trying out new adventures, gameplay and interface updates.These events are often insentivised with rewards for logining in during certain periods. The reward usauly arrives in an email to the staging in-game email that contains a code that is entered into the Voucher Claim slot on the TSO website will logged into your profile. Loading up the game you should receive the reward in a normal email.
Gifts tend to be in the form of premium resource and productivity boosts, such as the Cheese Sandwich.