I assume you are referring to the second Mrs. DeWinter (who is never given a name in the book...did you notice that?). A "pinnacle moment" is simply a high point, or important moment. For our unnamed heroine, her first pinnacle moment comes in her whirlwind courtship and marriage to Max DeWinter. She is suddenly liberated from her life as a "companion" to the overbearing Mrs. Van Hopper and feels, for the first time, desired as a woman.
The next pinnacle comes in her introduction to Manderley, specifically the breaking of the china cherub and its aftermath. This incident exemplifies how "over her head" our heroine is in her new life. She is completely overwhelmed by Manderley, and has no idea how to carry out her responsibilities. Further, she is frightened by the sinister Mrs. Danvers, who seems more the "woman of the house" than our heroine could ever be, and who personifies the lingering presence of Max's first wife, Rebecca.
The third pinnacle is the dance at Manderley, when our heroine falls prey to Mrs. Danvers and hurts her husband without knowing why. She feels that her marriage is a failure, without understanding her husband's reaction.
The moment when Mrs. Danvers urges our heroine to jump out of a window is a pinnacle in that it nearly causes our heroine to despair entirely. But she is saved by the next pinnacle moment: The discovery of Rebecca's boat and Max's confession. It is in this moment that she discovers that Max really loves her, and never loved Rebecca. She finally comes into her own as a woman, wife and mistress of Manderley.
The last pinnacle is the revelation of Rebecca's trickery (she conned Max into killing her) and the burning of Manderley. In many ways, she has lost everything, but she has finally fully gained Maxim as her partner and husband.