I haven't read the text for go specifically, but generally what that means is that the group releasing the software (google in the case of go) is saying that they won't sue you for infringing on any of their patents (that cover things in the software in question). So in this example it probably means that if google has a patent that covers something go does they are saying that they will not sue you for using go. Of course I'm not even close to a lawyer so take my explanation with a grain of salt.