When Simone Biles prepares for a routine, chalking up her hands, there's an incredible amount of pressure on her 4-foot-9 inch frame. A five-time Olympic Medalist, Biles is one of the best gymnasts of all time. At her first Olympic Games in Rio, she took home four gold medals and one bronze. She even has a move named after her: "The Biles." Commentators have called it "unreal." But for Biles, stepping onto the mat means shutting all of that out.

Gold medalist Simone Biles Alex Livesey | Getty Images

"Most of the time before I take the mat, I just tell myself, 'Confidence' and 'You have this,'" Biles tells CNBC. "It's like a daily reminder."

Her new book Courage to Soar reveals how the Olympic champion had to learn to believe in herself from a young age. She grew up in foster care. Later, she had to make the decision to forgo high school and the rest of "normal life" for training, and frequently she had to ignore critics and skeptics of her road to Rio. But those challenges ended up helping her.

"Every challenge I'd ever faced has made made me stronger and more determined," Biles writes.

Every challenge I'd ever faced has made made me stronger and more determined. Simone Biles Olympic gold medalist and professional gymnast