The Go Playground is 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.
There are limitations to the programs that can be run in the Playground. They must be single-threaded (but they may use many goroutines). There are also limits on execution time, and CPU and memory usage. The Playground can access only a subset of the standard library (notably absent are network and file system access). Therefore, the only communication a Playground program has to the outside world is via standard output.