In this document: https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/GuidedTour.html#//apple_ref/doc/uid/TP40014097-CH2
The type of parameter in sumOf are known as 'variadic' parameter.
The parameters passed are accepted just as a group of elements and is then converted into array before using it within that function.
A great example would be this post.
Passing lists from one function to another in Swift