In a previous post synchronous functions and iterators have been explained. Those functions are blocking, and in our daily busy life, we don't want our programs to block, we want a smooth experience. Well, if you are coding in C or C++, you should probably know pthreads, a library used to create threads, a way to create independent computation concurrently. The same exist in many other languages with sometimes different name, but doing mostly the same with just an interface and cool features.
Dart is working a bit like JavaScript, it uses only one thread to do all the computation by default, but this "thread" can be split in many small part allowing the illusion of concurrent or parallel computation. To make that possible, one need to create asynchronous functions, marked with async or async* keywords.
A function is asynchronous if its body is marked with the async or async* modifier. Otherwise the function is synchronous.
-- Dart Language Specification, Chapter 9, page 23
What does it really mean? Well, like the very great Dart documentation part about Concurrency explains, when a function is asynchronous, the event loop will listen for some changes in "background". When the function returns some values, the event loop receive it and return it to the caller. Let have a look on async keyword.
