The void is a visual representation of Eleven's mind while using extrasensory perception to locate other living creatures in different locations to her own.
The void appears as a vast endless expanse of nothingness. The floor is covered in a thin layer of water. It is completely empty, not including Eleven herself or whatever or whoever she is trying to locate.
Originally, Eleven could only enter the void while sensorily deprived, specifically when she was submerged in a sensory deprivation tank. By 1984, Eleven's powers had grown to the point where she could enter the void without subjecting herself to total sensory deprivation. Simply blindfolding herself was sufficient to induce the state. In this space, she can psychically locate people she usually can't reach under normal circumstances. Distance doesn't appear to be an issue, as she immediately found the Russian spy a continent away - she even found the Monster, which was in a different dimension entirely. It seems Eleven can choose to simply observe, or make her presence known by making contact, usually by "touching" with her psychic avatar. The people and objects she interacts with are not physically in the space, but are mental projections of their physical selves.
In 1983, an experiment was held at the Hawkins National Laboratory. The experiment was to test subject Eleven's abilities in eavesdropping over great distances. Tasked with finding a Russian spy, she was lowered into a sensory deprivation tank. She then entered a deep psychic state, allowing her to enter a mental void. With ease, she located the spy and was able to broadcast what he said over the lab's speakers. However, her focus shifted to a growling sound that came from some sort of creature. Frightened, Eleven fled and the experiment was aborted.
On November 6, Dr. Brenner decided to hold another experiment. Intrigued by the creature, he urged Eleven to make contact, saying it was reaching out to her. When she found the Monster, it appeared to be hunched over and feeding on a large egg. Cautiously, she approached and made contact. This caused a rift to another dimension to open and subsequently unleashed the Monster upon Hawkins.
Being unable to reach Will and Barbara under normal means, Eleven proposed to everyone that she could find them in “the Bath.” The group then came up with a plan to build a makeshift deprivation tank, setting it up at the Middle School. She first set out to find Barb, but unfortunately only found her corpse. The gruesome sight made Eleven panic and scream. After being comforted by Joyce, Eleven continued to search for Will. She found him hiding from the Monster in Castle Byers. Relaying Joyce's message, she told him to hold on and that his mother would be coming for him. However, she suddenly lost contact and Will vanished.
She primarily used it to visit Mike during their year of separation, and was also able to communicate with her mother, whose mind was otherwise destroyed rendering her incapable of speaking otherwise. After prompting from Terry, Eleven entered the Void to track and locate her "sister," Kali, in Chicago. Kali in turn used her to locate former employees of Hawkins lab to target them for assassination. Again, this was carried out through the Void.
Eleven learned of the dire situation in Hawkins via another attempt to check on Mike through the Void, prompting her to return to rescue her friends.
Behind the Scenes
The void bears resemblance to another void-like place in the 2013 film Under the Skin, where the main character, played by Scarlett Johansson, lures unsuspecting men. The Duffers have confirmed the film as being the direct inspiration for the void.
When creating the void, the Duffers decided to use practical effects. The effect was achieved with a thin layer of water on the ground, and having the surrounding area covered with black curtains. The Duffers explained they decided to go with this simple approach to avoid the story becoming too overblown with detail, while also using the the budget limitations to be more creative without the use of CGI:
"The Void was a great way to portray an abstract and sort of bizarre idea, but what we were trying to do with the show was go back to the idea that simple can be better. Even in our title sequence — as opposed to trying to one-up these huge shows or big movies with elaborate sequences, it was always about going back to the simplicity of something you couldn’t do before you had big CG effects."