What is this function call operator?

[Daniel Kozák via Digitalmars-d-learn]

On Thu, 27 Aug 2015 15:18:22 +0000
"Gary Willoughby" <dev at nomad.so> wrote:

> If you visit this link:
> 
> http://dlang.org/phobos/std_traits.html#isCallable
> 
> There is this paragraph:
> 
> "Detect whether T is a callable object, which can be called with 
> the function call operator (...)."
> 
> What is this function call operator? (...) Where can i learn more 
> about it?

http://dlang.org/operatoroverloading.html#function-call