trying to understand VHDL programs ..please help

i am a 3rd year student of ece dept...i m doing a project on this topic.....i am seeking someone's help to understand the VHDL program stepwise,which is listed below....
-- Design Name:
-- Module Name: ALU_Controller controls alu components bases off inputs
-- Project Name: DSD_Project (4 bit alu)
----------------------------------------------------------------------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity ALU_Controller is
Port( opcode : in std_logic_vector(1 downto 0); -- opcode and mode in
mode : in std_logic; -- mode
output : out std_logic_vector(5 downto 0)); -- all the outputs
end ALU_Controller;

architecture Behavioral of ALU_Controller is
type arrayRom is array (0 to 7) of std_logic_vector (5 downto 0);
constant Rom: arrayRom := ("000100","001010","000010","000000",
"000001","010001","100001","110001");

begin

output <= Rom(conv_integer(mode & opcode));
end Behavioral;

please help!!😕