Snippets are Layout Elements that you can drag n drop into the layout canvas. You add them to the Views/Elements folder of your theme and they have to end with Snippet.cshtml.
With this Snippet you can make a Page accessible only for authenticated users.
You have to add AuthenticatedUserSnippet.cshtml and AuthenticatedUserSnippet.txt files to your Views/Elements folder of your theme. After this you can drag the AuthenticatedUser snippet wherever you want and if the user is not logged ln will be redirected to a custom url or to login page.
Read more here