What is mutable const?

I didn't find any topics related to mutable const on SO. I have reduced code to minimal working code (on visual studio). If we uncomment //*data = 11;, the compiler complains about const-ness. I wonder how mutable const works.

class A
{
public:

    void func(int & a) const
    {
        pdata = &a;
        //*pdata = 11;
    }

    mutable const int * pdata;
};

int main()
{
    const A obj;

    int a = 10;
    obj.func(a);
}