From the Access and Permissions tab, you can decide that who can see a particular Block. This can be used in many ways:

• “Not Logged In” permission can be used to show a “Please log in” message only to site visitors who are not currently logged in.
• “Logged In and Authorized” can be used to show private content only to certain users or user groups who you authorize to view it.
• The default “Any” access means anyone can view this content Block.