Tess's mother shows a superstitious faith in the Compleat Fortune-Teller book by keeping it hidden under thatch in the outhouse, presumably lest it cast an unwanted spell on the house. She believes in its power, just as she believes in the power of the d'Urberville name (supposedly) having aristocratic roots going back to "Oliver Grumble's [Cromwell's] time."
As Hardy's narrator points out, this is part of the large social and cultural gap between mother and daughter. Tess, because of the newer education laws, has had the rough equivalent of an eighth-grade education in the national schools, and speaks the Standard English she learned there as well as the dialect her mother speaks. The narrator says there is a gap of two hundred years between the mother and daughter in education and outlook. Tess, he states, represents the Victorian age, her mother, the Jacobean.
More importantly, Tess's life will be badly compromised by the wishful thinking and poor decisions her parents, in their ignorance, make for her.