Reference of Reference in C++

I see code on StackOverflow every once in a while, asking about some overload ambiguity with something involving a function like:

void foo(int&& param);

My question is: Why does this even come up? Or rather, when would you ever have "a reference to a reference"? How is that any different from a plain old reference? I've never run across this in real-world code, so I'm curious as to what kind of code would need this.

It's a rvalue reference, Bjarne describes it here.

Shameless copying ("quoting"):

The rvalue reference

An rvalue reference is a compound type very similar to C++'s traditional reference. To better distinguish these two types, we refer to a traditional C++ reference as an lvalue reference. When the term reference is used, it refers to both kinds of reference: lvalue reference and rvalue reference.

An lvalue reference is formed by placing an & after some type.

A a; A& a_ref1 = a;  // an lvalue reference

An rvalue reference is formed by placing an && after some type.

A a; A&& a_ref2 = a;  // an rvalue reference

An rvalue reference behaves just like an lvalue reference except that it can bind to a temporary (an rvalue), whereas you can not bind a (non const) lvalue reference to an rvalue.

A&  a_ref3 = A();  // Error! 
A&& a_ref4 = A();  // Ok

It isn't a reference to a reference: such a thing does not exist.

It is an rvalue reference, a new feature added in the soon-to-be-standardized C++11.