There's a section called A Tour of Go [0] which has a series of code samples with explanations. The last part of the introduction to the tour says a little about what they call the Go playground [1]:

"This tour is built atop the Go Playground, a web service that runs on golang.org's servers.

The service receives a Go program, compiles, links, and runs the program inside a sandbox, then returns the output."

[0] - https://tour.golang.org/welcome/1 [1] - https://tour.golang.org/welcome/4

It's the same stuff that powers the playground: http://play.golang.org/

More info here: http://blog.golang.org/playground

It makes requests to a backend service. You can read more about the implementation of the Go Playground on the blog[0].

[0]: https://blog.golang.org/playground