what is exactly a rvalue reference in cpp?

rvalue_reference.cc

#include <utility>
#include <iostream>

int main() {
  int a = 5;
    int&& b = std::move(a);  //我觉得5-6行本就不应该编译过，b的类型是右值引用，但是b出现在表达式的左边，因此是个左值。这是为啥呢？
    b++; 
    std::cout << "a is : " << a << "\n";
  return 0;
}