What does =defualt mean? Myclass() = default;

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C++14

class MyClass { public: MyClass() = default; MyClass(const MyClass& src) = default; MyClass(const string& str) : mStr(str) {} // Move assignment operator MyClass& operator=(MyClass&& rhs) noexcept { if (this == &rhs) return *this; mStr = std::move(rhs.mStr); cout << "Move operator= (mStr=" << mStr << ")" << endl; return *this; } string getString() const {return mStr;} private:

What I have tried:

class MyClass
{
public:
    MyClass() = default;
    MyClass(const MyClass& src) = default;
    MyClass(const string& str) : mStr(str) {}   // Move assignment operator
    MyClass& operator=(MyClass&& rhs) noexcept
    {
        if (this == &rhs)
           return *this;
        mStr = std::move(rhs.mStr);
        cout << "Move operator= (mStr=" << mStr << ")" << endl;
        return *this;
    }
    string getString() const {return mStr;}
private: