A memoir is a book that someone writes about their own life, telling about things that happened to them and what they learned from those things.
Memoirs are usually interesting because they are about real people's lives and experiences.
Examples:
She wrote her memoirs to tell about her life.
I like reading memoirs about famous people.
His memoirs are about his time in the war.
The memoirs of the president were very popular.
In her memoirs, she talks about her adventures around the world.
I read the memoirs of a cat, and it was funny.