I've been out of the game since I graduated college in 2007, only now trying to learn and be certified in Java as the main language used in my degree was C/C++. So what the heck is "Go"? And how does it apply to the mainstream of programming that is already taken up by a billion other languages these days?...
There are indeed many different languages in use today---and I think that this will always be the case since so much software is being developed for so many diverse devices and application domains.
Go is a new language developed at Google and released under a very liberal open source license that makes it suitable for both commercial and non-commercial use. Go is another attempt at providing a successor to C (just as Objective-C and C++ are; and as Java is, as an attempt to succeed C++). Go provides a much higher-level approach to concurrency than C/C++/Java/etc., and has a very different take on object orientation (i.e., no inheritance).
I think that Go will be a very influential language because in many respects it has gone back to the fundamentals and rethought them.
It applies to mainstream programming just the same as any other language: if it suits your needs then use it! I would also add that Go is much safer and simpler than C or C++, and of course faster and with lighter syntax than Java.