I didn't read the link that was provided, and apparently you didn't either. I've contributed to a number of projects that deal with bar codes, including those for the USPS that appear on US resident's mail. The concept is relatively simple, and usually involves modifications that contribute to evaluating the correctness of the code in question.
I have some concerns regarding your ability to post, since you seem restricted to copying your previous input. I would despair at teaching you something beyond the concept of "2 + 2 = 4", or "See Dick run, see Jane run, see Spot run!". Perhaps you could ameliorate this by advancing beyond "Duh!".