It is a person that does a job so drab and uninteresting that it isn't worth giving a title or meaningful description. They work tirelessly and only garner mention when their boss screws up and blames the innocent functionary. They are blotters used to clean up messes.
Is this really a word that should be in a beginner's Spanish class? At this point I would rather learn about cashiers, waiters, bus drivers, tour guides, hotel clerks, maids, and police officers than ambassadors, functionaries, prosecutors, counts, kings, lawyers, judges, and authors.