Once you have filled out the form click the "check details" button and the details will be checked to check they are valid.
If they are not then any fields which are not valid will be flagged up for you to correct and then check again.
If all fields are valid then the fields will turn green, you will not be able to change them and the "check details" button will turn to a "register" button. Click this to confirm registration or you have the option to cancel and go back to edit the details.