Orlando is speaking to his oldest brother Oliver. He complains that Oliver is not treating him as an elder brother should, in particular that he is neglecting Orlando's education. Orlando states that he has no real options because the "courtesy of nations"--that is the custom of the society in which they live which is a general custom of the time--gives the land, titles, and power to the eldest brother. The technical name for this is primogeniture.
the courtesy of nations means the custom prevailing among the civilzed nations of the world. it allows oliver the inheritance of the estate of his father because he is the eldest son