Board admins can set the board's visibility by clicking the board's current visibility setting, to the right of the board's name, and selecting a new visibility setting.
On Workspace boards, admins can set the visibility to private, Workspace visible, or public. Enterprise Workspace boards have an Organization visible option.
When a board is private, only those users added as members to the board can view or edit it. Private boards which are a part of Workspaces are visible only to added members unless they are part of a Standard or Premium Workspace or Trello Enterprise—then, they are also visible to Workspace admins and Enterprise admins.
A Workspace visible board will be visible and editable by members of the Workspace by default. Workspace board admins can restrict members from joining and editing the Workspace board in the board's settings.
The Organization visible boards option is a Trello Enterprise feature that introduces another level of board visibility for Enterprise users. When a board is created or set as ‘Organization visible,’ it means that anyone who is a member of the Enterprise (on any Workspace that is managed by the Enterprise) can view the board, regardless of the Workspace the board is in.
A public board is visible to anyone on the internet, but not indexed by search engines such as Google. Only those users added as members to the board can edit, but any person with the link can view it, even if they don't have a Trello account.
Setting a board to public visibility allows all members of Trello to view your board and optionally vote or comment. You can turn other features (like comments and voting) on and off for public members via the 'Settings' menu.
Note: Boards within Workspaces that subscribe to Trello Standard or Premium will see additional privileges for the team's admins, such as the ability to see and edit all boards in a Workspace.