There are no specific requirements for applicants other than age (you must be > 15.5 years old at the start of the on-campus program), grade (you must be a rising senior or equivalent in your school system, i.e., entering your last year of secondary school before college), language (you must be proficient in English), and availability (you must commit to fully participating in the two-week online directed self-study program as well as the four-week on campus program).
However, successful applicants have likely taken (and aced) all of the advanced math and physics classes available to them as a high-school junior, have exceptional self-motivation and maturity, and have demonstrated a sincere interest in science as a career.