What is this function call operator?

[Gary Willoughby via Digitalmars-d-learn]

On Thursday, 27 August 2015 at 15:19:15 UTC, Adam D. Ruppe wrote:
> 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.

Sorry, I mean the three dots '...' that seems to be what the 
documentation is referring to. Also the `isCallable` template 
uses it.