This is what I found on it as I was curious as well.

Summary: One of the more frustrating aspects of the UFO phenomenon is its unpredictability. One of the mainstays of the scientific method is the repeatable experiment. In the case of an unpredictable and elusive phenomenon this is rather hard to organise. Occassionally UFO phenomena occurs in specific localities for an extended period. These are refered to as localised "flaps".

So the way I read it, a Flap is described by UFO Sightings or such in one area for a long period of time.