Private Reports are reports that are accessible to the report owner only (the user who creates the report) regardless of the user role or profile. The same also applies to Private Dashboards.
In Chapter 2, Concepts and Permissions in Reports and Dashboards, we explained that your admin may be able to access your private reports and dashboards. This is possible when the setting for Administrators Can Log in as Any User is enabled--the setting is located under the Login Access Policies setup. You can imagine this similar scenario with your office email, which can be accessed by the email server admin--they should not do this unless it is necessary and this activity is logged.
Users with the permission to create reports or create dashboards will be able to store the reports and dashboards that they create in their private folder. For reports...