A title list is a line of text that presents one or more tiddler titles, strung together with a space between each one and the next.
If a title contains a space, it needs double square brackets around it:
GettingStarted [[Discover TiddlyWiki]] Upgrading
Title lists are used in various places, including PermaLinks and the ListField.
They are in fact the simplest case of a filter, and are thus a way of expressing a selection of titles.