I believe that they were stories written for some newspaper/magazine at some point. They were 'lost' because they were never officially collated into a book.

70s 80s work pre colour of magic it seems

"A Stroke of the Pen: The lost stories by Terry Pratchett. But wait – a new collection of stories from the late and inimitably great Sir Terry? Perhaps this is my biggest title of the month. These stories were written by Pratchett in the 1970s and 1980s, while he was working for a regional newspaper, and have never been attributed to the Discworld author until now. They sound like a real mix of science fiction and fantasy, with topics ranging from time travel tourism to a visitor from another planet, and characters including cavemen, gnomes, wizards and ghosts. None are set in Discworld, but I, for one, can’t wait to hear the wry, dry humour of Pratchett once again."