Terrarium is a multi-language deployment platform based on WebAssembly. Think of it as a playground
for experimenting with edge-side WebAssembly. app 
doc
Being one of the first Fastly Labs projects, you can also think of it as our way of publicly experimenting with what the future of real highly-performant edge computing could look like.
When you click deploy in Terrarium, your code is being uploaded to a compilation cluster, which first converts the code into WebAssembly. Following that, we use our internal sandboxing WebAssembly compiler to generate machine code from it. And finally, that code is loaded into a web server and exposed over the internet.
Each of the requests to your code are sandboxed as they enter the server, so there should be no way for different requests to interact with each other and no way for your code to interact with anyone else's.
See also WebAssemblyStudio. github