Dao is ruled by Spontaneity

Dao is the cause of its own existence; its principle is Spontaneity ( 自然 Ziran ). To Cultivate Dao ( 修道 Xiudao ), one must follow Spontaneity. To be ruled by Spontaneity is one of the major characteristics and symbols of Daoism. How should we understand the notion of 'Dao is ruled by Spontaneity' ( 道法自然 Daofa Ziran )? 道法自然