Knowing about these is of little use to the everyday Tiddlymanual user.

The "Shadows" tab in the Index lists all of the Tiddlers that are 'built in', to make the TiddlyWiki format run properly - you can write over them but this will generally break stuff, so it is not recommended! (If you do by accident, then you can always just delete your local version and "our" original version which yours had overwritten will just "pop back" into place - so you can't break anything permanently!)

Mostly shadow tiddlers do the boring but necessary work of making the tiddlywiki run properly - but an example that you can (and generally do) change is, say, the SiteTitle.