Dear young one, an infidel is a word that some people use to refer to someone who does not believe in the same religion as them. In a book called "Infidel: My Life", someone is writing about their own life and how they became an infidel.
For example, imagine if you believe in a certain religion, like Christianity or Islam, and you think that it is very important to believe in it. Now, if someone else doesn't believe in that religion and thinks differently than you, you might call them an infidel.
In the book "Infidel: My Life", the author talks about how she grew up believing one religion, but then changed her mind and started thinking differently. She faced a lot of criticism and disapproval from the people around her because of this.
So, this book is really all about how someone can start to think differently than the people around them and face challenges because of that. But we should always remember to be respectful and kind to people who believe differently than us, even if it can be hard to understand their point of view.