Tracker provides an indexer, tag database, extensible metadata database, and a database for first class objects.
Ubuntu wiki discribes tracker in simple terms
In layman's terms, MetaTracker (commonly referred to as Tracker) is a desktop search application which allows you to quickly search your
documents, photos, music, and other files. The advantage of Tracker over the default search feature in Ubuntu (Nautilus -> Search for files...) is that results are returned much more quickly and all of the metadata (such as title, artist, track, album, composer, date, etc.) of the file is included in the search. Tracker also very small and efficient, consuming only abou 6 MB of RAM when indexing, and ~3 MB when idle.