Each user has a Universal Profile which includes name, date of birth, gender, mailing address, email address and phone number. Custom Profile Fields extend a user's profile with information that your organization wants to track. You have the option to add several types of fields and can configure whether the user can see or edit them and whether or not they're required or require administrator approval.
For example, if your organization requires a valid photo ID or certification prior to the registration, you can get the registrant to upload it here and make it "Required" before the user can do anything else. You may also add other field here such as additional contact / emergency contact information etc.
Click Add This Field to save your field. If you have made it required for the user, then the next time they login they will be immediately prompted to fill in that information.