What is Rust Staging?
There is two versions of Rust, the Release version - which is the common version that everyone plays.. and Staging - which is the development version.
The Staging version has all of latest changes, but might have bugs. The Release version is the most stable version - and at any one time should be the most bug free.
How to play?
You can install the Staging Version of Rust like the regular version. If you own Rust you should see Rust - Staging in your games list.
The staging version isn't compatible with the Release version, so you won't be able to join Release servers in the Staging version.