The registration opening tab is where you can set the date and time that the registration form will open.
At the set date and time that you have entered for registrations to open the page will automatically open.
Until that date you set, the page is not available for use and a message that you have entered into the 'Registrations not open display text' you can customs the message or go with the default - in this case it would be: 'Registrations are not open yet.'
Alternatively you do not have to select a registration opening date. If you choose not to set a date, the registration page will be open from when your event is approved.