In a function declaration, what's the difference between void fun(const string& a) and void fun(string a)?

Question

I know (correct me if I'm wrong) that const string& a is an lvalue reference and string a is a simple string variable, but I'd like to know if, in this context, there's a difference between the two, because in some tutorials I saw people using const [type]& var instead of the simple [type] var.

What's the purpose of using one of the two declaration?

Thanks