Passes all of an iterator's items as function arguments.

During evaluation of a function call, the runtime checks whether any of the arguments are (splay x) forms. If so, the form x must evaluate to an iterable. It's converted into an iterator; that iterator is repeatedly advanced until it's finished; and all of the resulting items are passed as function arguments.

(let ar '(1 2 3 4))
(prn (+ ..ar 5)) ; prints 15
(prn (+ 1 2 3 4 5)) ; prints 15

(splay x) is usually abbreviated as ..x.