Go to the Redeem link on the right hand side of the iTunes Store homepage. https://buy.itunes.apple.com/WebObjects/MZFinance.woa/wa/redeemLandingPage
Promo codes can be used to recieve an app for free. Once you have a promo code, usually given to you by a developer for PR/reviewing purposes, you can use it by opening up the iTunes app, going to "More" at the bottom, selecting "Redeem", entering the code then pressing "Go". You'll have to enter your account password and then the app will download onto your springboard just like any other app.
