When you create a new page it will automatically be set as Invisible. This is done so that you can create pages and sections and work on them over time without public view and then make Visible when you want.
Controlling visibility is a simple matter of clicking the page icon in the Visible column in the Management Panel:
Visible Icon
Visibility is also very useful in creating pages which you don't necessarily want to show in the website navigation. This may be special offer pages, promotion pages or pages for specific users from other website links or pages that you have built into the website's Design Template.