the & is for reference. In short that's something like a pointer, that can't be NULL. Wikipedia has something on this topic here.
References are cheap when they are used in function/method calls, since the object doesn't need to be copied in your function call. You still have the same syntax as if you had with a copied object. With pointers you would need to handle the special case, that the pointer is NULL.
That is a usual reason to use them. If I guess right and exchange means something like swap the tow objects x and y, the the cost of the function call is directly related to the cost of coping the object, so saving some copies may be a relevant optimization.
