Q. What happens if we add a new custom form question to User Registration and make it Required?

Admins can add additional questions to User Registration using Custom Forms (Admin > Settings > Forms), and then applying that Form to the Registration work-flow in general settings (Admin > Settings > General).

If the Admin chooses to make any new questions Required, then:

1. New users will be required to complete that question
2. Existing Users will be promoted to answer that question next time they login

If the question is not required - the users will not have to complete the question but can do so voluntarily.