A special code that you can share with others to get a free trial. You can access it from the My Profile screen.
That user will get a free trial. Your account will also get a free trial. Each trial is offered as a brand new license key, which you can find in My Purchases screen.
Every time someone accepts your invite code, you will get a new license key in your account. Each license key will start after a previously valid license key expires.
Suppose you already have a valid trial license key in your account, which will expire on October 20th. If someone accepts your invite code on October 15th, the new license key will have a starting date of October 20th.