A page can be restricted so that only specific users or user groups can view it.
To restrict a page that’s already been published, click on the three gray dots in view mode and select “Restrictions.”
In the restrictions dialog, select either “No Restrictions” or "Editing Restricted or “Viewing and Editing Restricted.” Enter names of the users or groups, then click on “Apply.”
To restrict a page when you create it, before it has been published, click on the little lock just above the page title to open the restrictions dialog.
Please watch our video Page restrictions - Learn Atlassian Confluence #82 which is part of the Video Tutorial Series "Learn Atlassian Confluence.
You may also read about Page Restrictions in Atlassian’s documentation.