A function is polymorphic if it accepts _arguments_ with different types (the return value is not considered). Thus
# let f x = x + 1;;
val f : int -> int = <fun>
is not polymorphic; it only accepts an integer.
Your function, on the other hand, is polymorphic. It accepts a two-tuple, where any type may appear on either position, e.g.
# let f (a,b) = "hi there";;
val f : 'a * 'b -> string = <fun>
So I can use f on both, say a string and a bool pair, or an int and an empty list.
# f("Blue Stilton", false);;
- : string = "hi there"
# f(42, []);;
- : string = "hi there"
/Martin