help understanding code

What is "zero" doing in this program.

use ieee.std_logic_unsigned.all;
use ieee.std_logic_arith.all;
ENTITY alu8bit IS
    port(a, b : in std_logic_vector(7 downto 0);    -- a and b are busses
        op : in std_logic_vector(2 downto 0);
        zero : out std_logic;
        f : out std_logic_vector(7 downto 0));
END alu8bit;
architecture behavioral of alu8bit is
begin
    process(op)
    variable temp: std_logic_vector(7 downto 0);
    begin
    case op is
        when "000" =>
            temp := a and b;
        when "100" =>
            temp := a and b;
        when "001" =>
            temp := a or b;
        when "101" =>
            temp := a or b;
        when "010" =>
            temp := a + b;
        when "110" =>
            temp := a - b;
        when "111" =>
            if a < b then
            temp := "11111111";
            else
            temp := "00000000";
            end if;
        when others =>
            temp := a - b;
    end case;
    if temp="00000000" then
    zero <= '1';
    else
    zero <= '0';
    end if;
    f <= temp;
    end process;
end behavioral;