You need to turn towards the end of this first scene to discover this reference and explore why it is used. Having been shamed and beaten by his brother, Orlando, Oliver then talks to Charles about what has happened in court. Charles tells him (and us as the audience) how Duke Senior was banished from his rightful position of leader by his younger brother, Duke Ferdinand, and how he has set up court in the Forest of Arden with some of his loyal friends that have deserted their positions of influence in the court to be with Duke Senior. Note how Charles describes them:
They say he is already in the Forst of Arden and a many merry men with him, and there they live like the old Robin Hood of England. They say many young gentleman flock to him every day and fleet the time carelessly as they did in the golden world.
Duke Senior is compared to Robin Hood in the way that he lives in the forest in opposition to the authority that is based in the court. In addition, he is said to be enjoying life and passing time "carelessly" as they did in the "golden world," harking back to a time of innocence and diversion when people were able to live lives free from the toil and strain of work and effort.