No one is allowed to copy the summaries, but here is an explanation of the time travel element. If you understand that better, the novel may be easier to comprehend.
Hannah is a young Jewish girl living in modern America with her family. At the start of the novel is celebrating the Seder meal and losing her patience with her family, their history, and all their stories. She is very much "a teenager." At some point in the evening, she throws open the door of their home and at that moment transported back to Poland in 1942. She knows she is Hannah, but everyone is calling her Chaya. Chaya is Hannah's Jewish name -- she was named after her aunt's friend who had died many years before.
The rest of the novel is the story of Hannah as Chaya in 1942. What is interesting about the narrative is that Hannah is always aware that she is from "the future" and that she "knows" what is happening because she has learned the history of the Holocaust in school. She lives the life of Chaya because she has no choice. As the story progresses Hannah loses the preciseness of her memories, but she still knows that she is from a different place in time. She recognizes the brutal reality of what is happening to her and the other Jews in the camp, and ultimately remembers the story of Chaya and fulfills that story.