[racket] there must be function right after parenthesis?

there must be function right after parenthesis?
hello~
while I am learning GUI programming in DrRacket,
I don't get how string literal can be placed right after parenthesis
such as
(define frame (instantiate frame% ("Example")))
(define msg (instantiate message% ("No events so far..." frame)))
(instantiate button% ("Pause" frame (lambda (button event) (sleep 5))))
(instantiate button% ("Ok" panel void))
I was thought one of the scheme grammar is there must be a function right  
after parenthesis
and I don't understand how such a code like
((func arg) arg2) works too.

could somebody please let me know how it works?
thanks in advanced
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