I've tried to grasp its description, but I am still not confident that I got the idea properly.
my doubt was is it related to Concurrent Garbage
As a simplified explanation - garbage collectors have two possible collection types, "incremental" and "full". Incremental collection is the better of the two for staying out the way, as it'll do a little bit of work every so often. Full collection is often more disruptive, as it takes longer and often has to halt the entire program execution while it's running.
Because of this, most modern GCs (including G1) will generally try to ensure that in normal circumstances, the incremental collection will be enough and a full collection will never be required. However, if lots of objects across different generations are being made eligible for garbage collection in unpredictable ways, then occasionally a full GC may be inevitable.
At present, the G1 full collection implementation is only single threaded. And that's where that JEP comes in - it aims to parallelize it, so that when a full GC does occur it's faster on systems that can support parallel execution.