You can think of TiddlyWiki as a database in which the records are tiddlers. A database typically provides a way of discovering which records match a given pattern, and in ~TiddlyWiki this is done with filters.
A <<.def filter>> is a concise notation for selecting a particular [[set of tiddlers|Title Selection]], known as its <<.def "output">>. Whenever ~TiddlyWiki encounters a filter, it calculates the output. Further work can then be done with just those tiddlers, such as [[counting|CountWidget]] or [[listing|ListWidget]] them.
A filter's output can change as tiddlers are added and deleted in the wiki. ~TiddlyWiki recalculates on the fly, automatically updating any filter-based counts or lists as well.