A token is a metadata name enclosed in curly brackets, for example '{Aperture}'. When the workflow is run, this will be replaced by the actual value, for example '5.6'. In the Text Node you can access all these tokens via the menus and the Insert button, but you can also type the tokens in directly yourself.

This is useful for generating text that will be different for each Version that is processed, for instance in helping make export sub-folders depending on rating/current date/any other metadata, copying metadata from one tag to another, creating naming systems that are more sophisticated than Aperture's internal ones (Aperture Assistant can use the last two digits of the capture year - '08' rather than '2008') etc.

Example - say you wanted to create export sub-folders based on image ratings and focal length. If you wanted the final sub-folder structure to look like:
Rating - 4/Lens - 15mm
Rating - 2/Lens - 15mm
Rating - 3/Lens - 70mm
etc.

You would enter 'Rating - {Main Rating}/Lens - {Focal Length}mm' and then add an Action Node with a 'add Finder sub-folder' action.

Note - metadata such as the focal length will be just a number, without the measurements such as mm, and the shutter speed will be a decimal figure not a fraction, e.g. 0.125 instead of 1/8th of a second.