Austrian philosopher Alexius von Meinong (1853-1920) coined this term to describe things that do not actually exist, but can be spoken of as existing. The term comes into frequent use during discussions of the philosophy of language and semantics.
A practical example of something to be found in Meinong's Jungle is the prime number between seven and eleven. No such number exists, yet it has just been clearly referred to in a grammatically truthful way. Another example is the statement "Santa Claus lives at the North Pole." While he could very well live there, there is no definitely existing referent to verify the statement; that is to say, Santa Claus doesn't actually exist (my apologies to the believers among you). Nonetheless, the statement itself is commonly regarded as being true, and so, according to Meinong, in some way, it is.
Therefore, all the implicit and unverifiable half-truths created daily by language have a happy home in Meinong's Jungle.