Discount Coupons vs. Customer Codes
The discount coupons are probably what you want to use.
You can create a coupon code that gives special pricing to some/all products in your. You can set valid dates for each code, a price range it applies to, free shipping, etc. Then when the customer is checking out, they can enter their coupon code and receive the discount.
The "customer codes" are similar to discount codes, but the customer enters their code before checking out (this requires a little special setup on the website). They then see their special product pricing as they are browsing your website.
You probably don't need to use the customer codes... the discount coupons are what most people use and are familiar with... i.e. codes for "20% Off Hats Until February 1" or "Free Shipping for orders over $40". Best of all, your website already has a box for people to enter their "Discount Code". It is part of "Step 1" of the order checkout process.
Manage discount coupons within the "Discounts & Coupons" section within the SEO-Cart administration back-office.