I just read Jonathan Sande's book about Dart. And found there an explanation of concurrency as a running code on one core, when parallelism execution on multiple cores. It is a "half-true" that may be useful for super-beginners who have Dart their first ever language, and have no CS background at all.
But concurrency is a pattern you separate your pieces of code to be able run them independently.
Here is a quote from Jonathan Bodner's "Learning Go" book:
Concurrency is the computer science term for breaking up a single process into independent components and specifying how these components safely share data.
I find it brilliant.