From Rosanne Cashriel:
This service will be available in Finder's Services menu and contextual menu if files and/or folders are selected. It enables you to add your own Spotlight keywords to a batch of Finder items. Look at it as an extension to the keyword feature in iPhoto. You can add as many keywords as you like. Your entries will appear in the Spotlight Comments field at the top of the Get Info windows of the selected Finder items. Typing a particular keyword in Spotlight will bring up a list of all the items tagged with it. Note that "Append to existing comments" option is checked by default - uncheck it to overwrite your previously entered keywords.