Go is a concurrent lang What does this mean?
doesn't this mean that it is a C/C++/Java.. alternative?
It means that it is a language with features suitable for concurrent (parallel, multithreaded, etc) programming. It has special languge constructs to support this type of programming. Concurrent programming can be done in other languages (C/C++, Java, etc) but it will (arguably) be harder and will probably result in more errors in the programs.
A concurrent language is a language that has language constructs for concurrency.
Go is a concurrent language because it has "goroutines".
Concurrency
Go provides goroutines, small lightweight threads; the name alludes to coroutines. Goroutines are created with the go statement from anonymous or named functions.
Goroutines are executed in parallel with other goroutines, including their caller. They do not necessarily run in separate threads, but a group of goroutines are multiplexed onto multiple threads — execution control is moved between them by blocking them when sending or receiving messages over channels.
Here are some resources about concurrent programming from some of the principal authors of the Go programming language.
Introduction to Concurrent Programming
Resources about threaded programming in the Bell Labs CSP style