Can someone tell me what Zeitgeist is all about?

What it mostly does is keep track of when you used which files and which applications. The Activity Journal application isn’t that useful yet (though you might sometimes want to know what file you were editing again at this and that time), but it can also be queried by other applications, which might enable extremely innovative applications. The best I can think of right now is e.g. your media player playing the kind of music you usually play when you’re editing a certain type of file (like work music when you’re doing work).

A quick Google search reveals that zeitgeist is a German term meaning “spirit of the time.” It refers to the moral and intellectual trends of a given era. For the Linux program, Wikipedia has this to say:

“Instead of providing direct access to the hierarchical file system like most file managers, GNOME Zeitgeist mainly classifies files according to metadata. This includes time and date of previous accesses, location of use (using GPS positioning), file type, tagging and more. In addition to local files, GNOME Zeitgeist also organizes web browsing history, email and other data sources.”