What is this function call operator?

[Adam D. Ruppe via Digitalmars-d-learn]

On Thursday, 27 August 2015 at 15:18:24 UTC, Gary Willoughby 
wrote:
> What is this function call operator? (...) Where can i learn 
> more about it?

It is the () at the end of a thing. You can overload it in a 
struct by writing an opCall member.