"Default tiddlers" refers to the list of tiddlers that are opened at startup in the browser if the address bar does not include a permalink.

There are two ways default tiddlers can be defined:

• A title-list eg: TiddlerTitle and [[Title with spaces]]
• Filter expressions, using filter operators eg: [tag[HelloThere]]

The resulting list of titles is then inserted into the story river.

The control panel -> Info -> Basics -> Default tiddler setting includes a text box for direct access to $:/DefaultTiddlers.