It's a unique, one-time-use code that allows you to instantly create an account with any desired username that you want. You choose your username ;-). Once you redeem the code and choose your username, you're presented with your master keys and that's it! Instant magic. It requires 3 Steem to create an account and once you create the account, the created account has that 3 SP. You're immediately up and ready to roll!