Potiphar put Joseph in Pharaoh's prison.  Later, two other prisoners joined Joseph in prison.  The prison was watched by guards (art not shown because it doesn't match the verse).  While in prison, Joseph interpreted these men's dreams.  Years later, when Pharaoh had a dream he wanted interpreted, one of these men told Pharaoh about Joseph.  Joseph interpreted Pharaoh's dream and was put in charge of saving food for a future famine.