There are many things we can learn from this novel. When given a question like this, look at the themes, for these are the messages that the author wants her readers to take from literature. Also, since reading is personal, ask yourself after reading a book, what the characters and events mean to you.
The major theme is how colonialism affected the lives of the people in the colonized country. We learn how Europe and the U.S. exploited Africa's resources, taking the gold and diamonds while forcing the native people into labor. We learn from these historical events that we should not let this happen again and that we must get involved so it doesn't happen again. We must learn from our mistakes in the past to protect the future.
We also learn what happens when one culture tries to force its beliefs and values on another culture. One country's ideas of religion or politics doesn't work for another country, especially when it already has its own rich, traditional customs and values.
Family relationships are also explored in the novel. Nathan is willing to endanger his whole family because he's mentally disturbed due to his experiences in the war. It takes the death of her child before Orleanna has the strength and courage to get the rest of her children out of Africa.
You should also write down what you liked about the book and what you feel you got from reading it. What life lessons do you take from it? That's how we learn the real value of reading literature.