C++ :: Explicit Conversion From String To Class Reference?

following code that I'm reading out of the book "The C++ Standard Library".

class C
{
public:
explicit C(const std::string & s); // explicit(!) type conversion from strings.
...

[Code].....

Now I understand that they are saying that an explicit conversion isn't allowed but what I don't understand is what explicit conversion would be happening if there was one allowed.

Specifically I don't understand the const C & elem syntax. How does this work since the collection holds strings. What would be the syntax for how this:

const C & elem

gets strings. I was thinking it was a class reference that someone how converts to a constructor function pointer or something but i'm really confused.