This is a classic sign up coupon. You sign up on the manufacturer's website whereupon they can pester you with emails to try and sell you more and in return you get $3 (USD - this is an American promotion) off a "first bag of Instinct Raw, grain-free Instinct Kibble, or wholesome Prairie Kibble". That's it. If you want to save a few bucks, go for it.
This is the sign up page. I believe that this lasts until the end of May 2012.