At Admin Panel -> Settings -> Switches, there's a switch called 'quick registration'. When this switch is off, the member registration page is long with lots of optional fields. This long form can reduce the rate at which people register, so the "quick registration" switch allows you to instantly simplify it into a very short form with just the basic necessities. When quick registration is turned on people will only be prompted for a username, password and email when they register. All other fields will use the values set on your Admin Panel -> Members -> Registration Defaults page. Of course, the member can use their Edit Profile page to change all the details later after they've registered.