The Void is a another plane of existence where Janet resides. It is assumed that each Janet has an individual Void that corresponds to that Janet. It is possible that Bad Janet and Neutral Janet voids differ in appearance from the Void of a Good Janet.
The Void is a place, or more accurately a non-place, where Janet resides. It is described by her as a 'boundless, barren nothingness'. When she created Derek, it is where both of them lived together until Derek moved out.
Janet can move between The Void and her assigned neigbourhood at will, but does not have the power to move directly between The Void and other locations such as The Medium Place. However, she can move between her Void and the Neutral Zone, most likely because there is a backdoor to the Neutral Zone from the Void.
For a time, Derek was stored in a "corner" of Janet's void within a crate, after being put in "power saver mode". This implies Janet can easily traverse huge distances within her void at will, though it still takes some time to find him. However, we can hear Janet as she searches for him, implying that space in The Void operates differently than it does in the rest of the Afterlife, just as time operates differently in the Afterlife than on Earth.
In Chapter 35 "Don't Let The Good Life Pass You By", Janet, while making contact with a portal to Neutral Zone (and therefore being able to use her powers) transports the four humans and Michael into her void, effectively killing the humans on Earth.
In Chapter 36, "Janet(s)", Janet brings the humans with her into her void. Their physical essences deteriorating, the humans take on the form of Janets, so Janet gives them their own signature clothes (which Gen also gave them when they visited the Neutral Zone in The Burrito) to help distinguish them. Janet created a living-room-esque location for the humans within her void to reside so that they would feel more comfortable, but Chidi swapped it out for a classroom so that he could teach Eleanor, Jason and Tahani about the concept of "self". When Eleanor loses her sense of self after realizing she's acting vulnerably, she begins to destabilize the Void until Chidi reminds her who she is, returning the humans to their forms and causing Janet to throw them up.
The Void seems to be the essence of Michael's Janet, as there are multiple side effects that Janet experiences when she brings the humans into her Void. When Eleanor conjures a puppy, Janet finds herself burping an auto-tuned song. When Jason conjures a hot tub and a version of Pillboi (who is aware of his own conjuring, as he says, "I was just chilling being nothing, and then I was!" and when Janet discorporated him, "Oh, dip! I'm not a thing!"), Janet states that "my molecular essence is fragmenting and it's giving me the worst headache". When Eleanor loses her sense of self, Janet's molecular essence does destabilize, as she is shown to have glowing white lines tearing her apart, a mirror of what is seen in the void (chunks of its existence disincorporating). When Eleanor retains her sense of self, the four humans regain their physical forms. However, Janet cannot withstand their presence in the Void, so she throws them up and they appear in the Accounting Department alongside Neil, Janet and Michael.