Did you take math in school???
n can be any natural number. Meaning, that it can be any number without desmos or fractions which is bigger than 0.
In other words, n is a variable.
It is the number, which you put into the brackets, when you call the function (when you apply the prcess).
I strongly advise reading this tutorial:
http://www.ibiblio.org/obp/thinkCSpy/chap03.htm (functions in general)
and
http://www.ibiblio.org/obp/thinkCSpy/chap05.htm (useful functions and how to apply them)