A liquid substance containing the vitality of the living thing it came from. When consumed, it has age-reversing effects, although the lasting properties of the essence depend on the race of the victim--Gelfling essence lasts for a long time, while Podling essence only lasts for a few seconds.
A being drained of essence is completely without willpower or energy, and is easy to order about, making a perfect slave, as the Skeksis soon discovered. They also have withered faces and empty, milky eyes.
When SkekTek discovered how to drain essence, the first victims were Gelflings, but after there weren't any more left, Podlings were used.
The process of draining essence is as follows: the victim is restrained in wormscrew clamps. The most disturbing form of restraint occurs by the clamp used to forcefully position one's neck so that the individual is not free from moving its eyes from the powerful beam of the crystal. The eyes are forced to stare into the the Dark Crystal. Their essence leaves their body and flows through a tube to the side, where it is collected in a vial for later consumption. Only the Emperor is allowed to drink essence, although early in the Skeksis' existence, when it was easier to come upon, all of them partook of it.
Being drained of essence and made into slaves was the ultimate fate of all captured Podlings, and when Kira was captured by the Skeksis, she was going to have her essence drained. However, only a few drops of her essence were collected before the animals in SkekTek's laboratory helped her escape.