If you are flying to or from a country that operates the Advance Passenger Information System (APIS) (see https://en.wikipedia.org/wiki/Advance_Passenger_Information_System), you see a message on the Reserve screen indicating that you need to enter this information:
Airlines have the right to refuse boarding if this information is not provided.
Entering this information in the booking tool avoids wasting time at the airport and potentially delaying the flight.
Click the link in the message to display the form to enter or update your details, some of which are pre-filled from your user profile. Travellers who have more than one eligible passport in their profile can choose which passport APIS information to add to the booking.
When the details are complete, click Validate on the form, then select the check box that indicates your acceptance for transferring this data to the airline.