What is the significance of the title of The Hundred Secret Senses by Amy Tan?

The hundred secret senses of the title relates to the narrator's half sister, Kwan, who is able to see "yin people" as she calls them, or helpful ghosts. What is so fascinating about this character is that she accepts the presences of these "yin people" as part of her everyday, normal life. Lots of these indidividuals are the spirits of Kwan's family and friends from her past lives, and thus there is an inter-relationship between the past and present that is constantly stressed. Note how the narrator refers to this aspect of her half-sister:

My sister Kwan believes she has yin eyes. She sees those who have died and now dwell in the World of Yin, ghosts who leave the mists just to visit her kitchen on Balboa Street in San Francisco.

"Libby-ah," she'll say to me. "Guess who I see yesterday, you guess." And I don't have to guess that she's talking about someone dead.