This is the total number of shares your Corporation will ever have.  This number is stated in the Articles of Incorporation.  You can only change this number with the approval of a majority of shareholders and by amending the Articles of Incorporation.  Authorized shares do not have to be distributed.  When authorized shares are distributed to the shareholders, they become "issued" shares.  Most Corporations usually authorize a higher number of shares than required in order to be able to issue stock in the future.