The "myth of pre-history" is an idea used by some scholars of indigenous, non-European cultures. It is an idea that is used to combat the Eurocentric idea that only Europeans (and possibly a few others) were truly able to create history and civilization.
Scholars who talk about this myth argue that Europeans see other cultures as backwards and primitive. Europeans see cultures that did not develop literacy and, thereby, written histories and they say that those cultures are primitive and less developed than those (largely European) cultures that did create written histories.
By calling this the "myth of pre-history" scholars are arguing that pre-literate societies that had no written history were not more primitive than other societies. They argue that this myth has simply been created as a way of justifying European dominance of other cultures.