The registration closing tab is where you can set the date / time that the page will automatically close.

From then onwards, the page is not available for public use and a message is shown on screen - 'Registrations Closed' - we generally recommend that you do not use 00:00 please use either 23:59 or 00:01

The text box allows you to type a message which is then displayed on the screen to provide them with further information once the page is closed.