Kong is a noun and adjective in the Chinese language that has the basic meaning of "unoccupied" or "empty\emptiness". It is an important word to know, because it is present in such compounds as "kongqi" (the term for, literally "empty air"), and is also used in day to day conversation to signify that someone is free to do something. If you ask someone to do something, you may ask them if they "have kong", that is, whether they have some unoccupied time to go and eat lunch and the like.
Along with being a rather handy all around term, kong is also a technical term in Buddhism that means the Void, with a capital V. As such, it is usually contrasted with se, or form, although the Heart Sutra famously states : Kong ji shi se, se ji shi kong "Emptiness is actually form, Form is actually Emptiness". What exactly Void means in a philosophical sense, I will leave it to the reader to decide. However, I would say that kong corresponds to the Indian term Sunyatta and to the Hebrew Ayin or Kether.
With that rough correspondence being said, it should also be said that it is part of the charm and genius of Chinese mysticism and Chinese culture that one term can be used to cover both the concept of being free to eat lunch, and being able to merge with the Grand Ultimate.