It simply means an Unidentified object - at the time of that sighting in the sky.
However when the object remains unidentified --- then it starts to have implications, especially when it belongs to an ever increasing number of such sightings...
If you wish to follow up on this, then...
I would recomend watching the DVD "The Silent Revolution of Truth" which is an excellent summary of the Billy Meier UFO contacts with the Plejarens. The various evidence are presented, plus an overall summary. While you can view much of this on various sites, etc - it is altogether here - plus some new material.
I strongly recomend
www.figu.org
(English Discussion Board) and reviewing the various discussions there - particularly the Questions and Answers to/from Billy Meier, in which straight answers are given. For example a confirmation on which contactees are genuine, and which ones are hoaxed/faked.
There has been deliberate attempts to present UFOs in a negative and fearsome light - such that various UFO sightings are not of ET origin, but terrestrial.
Over the past 5 years I have downloaded all I can about Billy Meier, and also all the UFO documentaries I can find - plus documentaries which deal with ancient civilisations and their technologies, astronomy, science, etc etc --- also particularly those which deal with strange and weird stuff...
The Disclosure Project material is worth looking at, which seems to confirm what Colonel Corso was saying all along... Likewise Linda Moulton Howe's "Exopolitics" videotaped lecture...