The word 'megafauna' means big (mega) animals (fauna). In the context that we are using the term, it means Australian animals that collectively died out in a mass extinction about 46,000 years ago. They were very large, usually over 40kg in weight, generally at least 30% larger than any of their extant (still living) relatives. Although many of them were marsupials; including giant kangaroos and wombats as well as other strange beasts like the marsupial lion, there were also huge snakes, lizards and birds in ancient Australia.
These animals had existed here for around 11 million years, then within just a few thousand years, most of these giant animals disappeared. There are several theories to explain this relatively sudden extinction and the discoveries at Mammoth Cave have helped with explaining their disappearance.