The documents due for release are witness reports of apparent UFO sightings, many by civil pilots and military personnel. Most were simply collected and filed by a small, secret unit within defence intelligence called DI55. A few are thought to have been investigated further by the military, but the details have never been made public. There are 24 files due for release, each containing 200-300 reports of sightings, plus internal MoD briefings and correspondence.
However, this is usually a combination of wanting to appease public pressure by releasing only what the public have reported in the first place rather than the detailed MoD findings and secrets on this subject. After all, a secret is still a secret!
Or, as some are already suspecting, it is part of a global acclimatisation program for readying the general public into acceptance of the UFO presence for more gradual public disclosure. Whatever the reason, you can be fairly sure that nothing of importance will be released without official sanction - and most probably not in these upcoming files...
Still, its a start.