The term ‘punch above one’s weight’ means performing or achieving results better than expected and beyond one’s ability, skill, experience etc.
Example of use: “Although Brad isn’t the best track runner, I think that if he trains hard and punches above his weight, he’ll be able to qualify for the upcoming competition.”
In the early 19th century, weight classes in boxing were established and boxing matches were divided into different weight categories -- flyweight, lightweight, etc. Since the regulations were established, only boxers of the same weight category could fight each other. When a boxer from a lighter weight category fought someone in a heavier category, he'd be 'punching above his weight'. Today the term is used figuratively in occasions where someone tries to compete outside their comfort zone and usual class.