What is a a C++11 extension [-Wc++11-extensions]

Hi I need some help understanding where this error is occurring,

warning: in-class initialization of non-static data member is a C++11 extension [-Wc++11-extensions]

and this is the section of the code that it is coming from:

typedef struct Hand {
    bool straight = false;
    bool flush = false;
    bool four = false;
    bool three = false;
    int pairs = 0;
    } Hand;