Goroutines/"fibers"/"green threads" are usually scheduled by the runtime system across a small pool of actual OS threads.

I thought it was obvious from context: OS threads are too heavyweight for fine grained concurrency

Go uses userspace threads. It’s also interesting that Go and Java are the only mainstream languages to have gone this route. The reason is that it has a huge penalty when calling FFI of code that doesn’t use green threads whereas this cost isn’t there for async/await.