A force of nature is someone who seems as strong and energetic as a bolt of lightning or a powerful storm. A basketball player who sinks an astounding number of baskets is a force of nature.
This phrase is often used admiringly: "I don't know how she accomplishes so much in one day — what a force of nature!" The implication is that people who are forces of nature exceed normal human abilities and seem to be fueled by something more powerful, like shifting tectonic plates or the ocean's tides. You can also use force of nature in a literal way, for forceful natural occurrences like storms and earthquakes.