To whom is the "secret life" referring? In other words, what are the secrets and to which characters do they relate?

Just as the bees live a secret life inside the hive, a
life that people on the outside cannot see, many of the characters in "Secret Life of
Bees" live their own secret lives. When Lily first meets the Boatwright sisters, she is
surprised that they are intelligent and cultured, because they are black. She
says,

“T. Ray
did not think colored women were smart. Since I want to tell the whole truth, which
means the worst parts, I thought they could be smart, but not as smart as me, me being
white. Lying on the cot in the honey house, though, all I could think was
August is so intelligent, so cultured, and I was surprised by this.
That’s what let me know I had some prejudice buried inside me” (pg
78).

She is prejudiced
against them, even though she tries not to be. Once she can see into their lives by
being a part of it, she realizes the complexities of who they are. Lily, as the
protagonist, has many secrets. She hides the reason she has come to the Boatwright's
house, not telling August about her mother. Lily is lost through most of the novel, and
is searching for answers to  secrets about her mother and what happened to her in the
months before she died and leading up to her death. It is a personal quest for Lily, to
come to terms with her mother's death and accept that there are those who love her. She
keeps secrets -- secrets of her past, of her thoughts, and of her feelings -- from
everyone, but until she can talk about these things, and bring them out in the open, she
is unable to grow and learn the truth.