Click the referral code to be taken to the sign-up page with the code pre-entered for you:
Using a referral code when signing up for Star Citizen gives you an additional 5,000 UEC (in-game currency) when you start. A $5 USD value.
Either use the code above, or if you prefer, use the referral code randomizer. Either way, remember to submit your own referral code to the randomizer once you've created an account.