It is the plane of ultimate Chaos and randomness, where matter and thought collide, allowing for infinite possibility.
Though unlike THAT other realm of Chaos that we all know about, Limbo is FAR less hazardous to your health; your physical body will not dissolve on entry and neither will your soul immediately be twisted into something to serve the whimsy of Dark Masters.
Geography and Traits
As a rule, Limbo does not have a discernable geography. The plane is primarily made up of "Chaos Matter" which is an endlessly roiling and toiling stuff of no constant physical substance or appearance.
A pocket of air might suddenly congeal into water, harden into stone, then spontaneously ignite into flame, before bubbling into water again... There's no particular pattern to what might happen next, and it might give you the impression that Limbo is inhospitable.
Thankfully, Chaos matter can be shaped into something else merely by willing it into something else and there exists guilds of "Anarchs" who spend their days doing just that. At the most basic level, any person can create a pocket of stabilised air for them to breath without any fuss. More complex combinations of elements can be created with stronger minds to hold it together, allowing individuals or groups to create virtually anything imaginable and imbue their creations with any trait they can conceive of... From a field of rainbow unicorns to a river of boiling piss.
This makes travel fun, as areas dominated by "Air" allow users to fly (or fall) in a chosen direction, where swimmers could create "Water" for themselves and move that way. You could also go more complicated and build an imaginary road for yourself if you simply wanted to walk somewhere.
Those areas of controlled Limbo last as long as the creator maintains his interest, or unless someone else is around to maintain control for him. Otherwise it all just melts back into chaos matter, so woe betide anyone who falls asleep in his stabilised area without someone to keep watch.
The most powerful minds can imbue a level of permanency on the chaos-stuff, so they don't have to worry about it dissolving when they stop paying attention. Nor do they need to be concerned about someone else's idle thoughts punching holes in it accidentally, so it is those areas of stable matter that population centers can most often be found.
However, it should not be understated that the plane is Strongly aligned to Chaos, so if you're a structured, well ordered individual (i.e "Lawful") then your psyche might crack under the pressure of all the nonsense that the plane puts out, coming with a corresponding reduction in your ability to control the environment.
While the laws of physics are always in flux, the laws of magic are equally affected: Spellcasting becomes hazardous in all areas other than those permanently stabilised, creating a chance that any time you cast a spell it might fly off in a random direction, do nothing at all, do something entirely different, or go Nuclear. That's just the nature of Chaos, so don't say you haven't been warned.
Limbo has three major food groups:The Petitioners, the Slaad and the Githzerai.
Limbo Petitioners are those who came to the plane as their reward in the afterlife, their souls take on the form of self-controlled chaos matter in death and typically have no sensible shape or appearance, they are as equally likely to exist as a talking cloud one minute or a twisted knot of rope the next, then set you on fire for the privilege of eating with their spoons (?) ... thankfully they are quite rare, for a few reasons. Firstly, because it's a bizarre individual who solely embodies the alignment to THAT degree of Chaotic Stupid to end up in Limbo. Most Chaotic Neutral individuals end up in Ysgard (If they were hedonists or thrill seekers) or Pandemonium (if they were nihilists or "actually" insane). The second reason is that in a realm of infinite space, a creature of pure chaos, uninterested in the affairs of others, is likely going to strike off on its own and never be seen again.
The Slaad are the "Natives" and embody Chaotic Neutral as much as Archons and Demons embody their own alignments.
Githzerai are cosmic immigrants from a long time ago, they could've picked anywhere in the multiverse to settle down but they chose here for the sake of a good challenge and started stabilising the environment to suit their needs.