f(qID,{loopiter1,loopiter2,...,loopitern})
returns a form object of some kind. A form is a question or loop node in a survey, and the argument qID is the question ID as defined in the properties of the question.
The rest of the argument(s) (loopiter1,loopiter2,...,loopitern) are only used for questions inside loops. These arguments are codes of specific loop iterations, starting with the innermost loop if there are nested loops (loops within loops). The curly brackets {} are used here to indicate that the loop iterations are optional.
Let us say you have a question q1 in a nested loop structure with two loops. The outer loop has iterations with codes "1" and "2", and the inner loop has iterations with codes "a" and "b". Then the question q1 is asked four times, and these four instances of q1 can be referenced like this (in the order they were asked) from outside the loop:
f("q1","a","1")
f("q1","b","1")
f("q1","a","2")
f("q1","b","2")