It links your new account to the partner who referred you — if you don't have a code, leave the field empty.
On the sign-up form the field is called Referral / Affiliate code and it is marked (optional). The note underneath says the same thing: if a partner referred you, enter their code here. Leave it blank and registration goes through exactly as normal, with no extra step.
If you arrived through a partner's link, you don't need to type anything: the code from that link is remembered in your browser for 30 days and appears already filled in when you open the Create account form. You can clear or change a pre-filled code — whatever is in the field at submit time is what gets recorded.
A code may only contain letters, digits, hyphens and underscores, up to 64 characters; anything else and the field flags it. If you're not sure you have the code right, ask the partner who sent you the link for the exact one.
This is not a discount or promotion code — it only records which partner your account came through. It is also unrelated to the invite link used to add colleagues to your team; that is a separate flow.
Also asked as: Is the referral code required? · I don't have an affiliate code, what do I enter? · What is the partner code on the sign-up form for? · Can I leave the referral code blank?