Incunabula refers to a book printed before 1500. The derivation of the word incunabula is Latin, meaning "in the cradle". This term is used because printing using movable type was in its infancy at the time.
Johann Gutenberg printed the Gutenberg bible in Mainz, Germany in 1454-55, the first book printed using movable type in the West. Between 1455 and 1499, inclusive, about 40-50,000 titles were printed, totalling 8-12 million books. This is about as many books as were created in the West in the 1150 years prior to 1455.
Incunabula is also used to refer to manuscript books created before 1500.
I do not want to get into the discussion of exactly what Gutenberg did here. That is better addressed in the Johann Gutenberg node or Gutenberg Bible node.