Will-O-the-Wisps, corpse candles/lights, ignis fatuus, corposant, jack-o-lanterns, fetch candles/lights, earth/ghost/witch lights...all these are names for basically the same phenomena, that of floating lights with no discernible source or carrier. However, while there are a multitude of names for this phenomena, there actually tend to be certain trends observable across the different names, and those trends form the meat of our discussion today.
So, what is the general meaning attributed to such phenomena? Well, this seems to fall into two major camps. One is that it's a phenomena that forecasts death at a given location, with the type of death warned of being dependent on the color or number of the lights. Usually the death forecast is that of the viewer or the viewer's loved ones, with small blue lights indicating a child's death, larger white lights for that of an adult, and a number of lights for a mass death, thought there have also been reports of lights in most other imaginable colors. It is important to note that in this particular framework, the lights in and of themselves are not sentient and therefore not inherently moral or malevolent, they are simply an impassive warning of a fate yet to pass. In this manner, while they are indeed unexplained phenomena, many attributed their viewing as a sort of divine intervention that allowed them to say their goodbyes to any affected. Also worth noting is the virtual impossibility of arguing with or avoiding this sort of phenomena (this sort also tend to be called the corpse/fetch candles/lights), as again, they are merely warnings and not direct enablers or participants in the fate of the viewer.
The second main school of thought holds that the lights are ghosts themselves, but the intent of these beings varies from culture to culture...though the general consensus is that it is wise to stay away (in fact, "ignis fatuus" literally means "foolish fire", named such after those who'd chase after it). Basically, the intent of the entities falls into two sub-categories. The first, and most benign -and- rare, is that of "unfinished business". In this case, the ghost is helplessly trying to signal any passerby to a particular area in the hope of them completing whatever task keeps them from crossing over. The most usual occurrence in this camp is that of a death in some far-flung area quite distant from the trappings of society (possibly also involving bringing a murderer to justice, if such a death wasn't accidental), and the entity's need for a proper burial for their remains, and according to some legends, they may possibly even reward the one who helps them by directing them to a treasure afterwards...though getting to the light in the first place can prove quite a challenge, as usually the terrain the first victim died in had to be a requisite level of treacherous to have caused them to die unexpectedly in the first place, so the task is not without risk.
Now for the other, more common sub-category. This sort is particularly malevolent and insidious, with the "nicest" of them causing travelers to become incredibly and hopelessly lost. More often than not, these lights are quite hypnotizing to look upon, overpowering the will of lesser individuals and leading them to their deaths. These entities are reasoned to be sinners that were for whatever reason denied entrance into heaven or hell, and are thus stuck on earth for all eternity, and that the boredom of such a fate has led them to treat us "fleshies" as playthings by tormenting us until such a time passes with which they get bored, at which point they end the "game" and consequently, the victim's life. It is worth noting that while there are supposedly a variety of ways to avoid and repel the spirits of this class, there are no direct ways to fight them.
As far as natural explanations go, these lights are usually reasoned to be fireflies or the ignition of swamp/carrion gas, but their occurrence across cultures with the same relative explanations leads one to wonder if there isn't something more to the tale of the mysterious lights...
