Default Page Permissions
Pages within the Wiki are generally open to anyone who has access to the Jira project the page belongs to.
In fact, all newly created pages can be seen by anyone with access to the project. Normally, you would then change page permissions on a per-page basis whenever you need.
In some instances however, you might have a scenario where a specific group of users is responsible for maintaining pages, or you want to exclude some users from being able to do certain things by default.
To accommodate that, the Jira Cloud version of Team Pages allows an admin to set default page permissions.
This works the same as settings permisssions on an individual page, except that these permissions are automatically added to newly created pages. Thus, if you use this setting, new pages do not start as open, but instead start out as locked.
For reference, we also have some notes on who can do what with pages.