This is a powerful little novel about the horrors of the Holocaust in 1942 Poland, and there are many possible themes to discuss, but two that I think are especially important and which ultimately tie to one another, are the ideas of survival and memory.
The novel opens with a modern-day Hannah at the family Seder dinner. She is bored and disinterested in the event and takes the prayers and the rituals in a rather blase manner. She feels she has heard all the stories too many times and they clearly don't have much of an impact on her anymore. She is annoyed by her family and just wants the night to end. It is at this point that the sci-fi time-travel piece of the story hits us. Hannah is "transported" back to Poland in 1942. She is a young Jewish girl name Chaya (which is Hannah's Jewish name) and she is almost immediately immersed in the tragic story of the transport trains throughout Poland that rounded up whole villages of Jews and took them to the concentration camps for forced labor and eventual execution. Hannah has all of the memories from her education on the Holocaust and knows exactly what is happening, but she is rather helpless to stop any of it from happening. She is terrified because she is so out of place from her "normal" life as a teenager in the United States.
But she quickly learns about the importance of survival. She learns the tricks of the camp and how to avoid being "chosen" for the ovens each day. She learns how to make alliances and deep friendships with the others so that together they can do what they need to in order to survive. She learns how to hide the children, get a little more to eat, stay out of trouble, etc. She learns these skills from her aunt, Gitl and other women in the camp, such as Rivka.
At the end of the novel, she realizes that she has lived her life of the future in America and has many memories, so dying wouldn't be as much of a loss for her as for others, so she switches places with Rivka as Rivka was being taken to the ovens. We learn at the very end of the novel that Rivka survived the camps and moved to the United States -- she is Hannah's Aunt Eva. Hannah's Jewish name is a tribute to the young woman, Chaya, who died in Rivka's place so that at least one of them would survive another day.
The ultimate message of the novel is that the world should never forget what the Nazi's did during the Holocaust. The survivors will eventually die along with their memories, but the stories and the memories of the horror of the Holocaust should always be honored and respected so that nothing like that will even happen again.