Short answer: A catastrophic plan has very low premiums and a very high deductible, and it’s limited to people under 30 or those with a hardship/affordability exemption. It mainly protects against worst-case medical costs.
Catastrophic plans are a special ACA category with the lowest premiums and the highest cost-sharing; you pay nearly all routine costs until you hit a high deductible (set at the year’s out-of-pocket maximum), after which the plan covers essential health benefits. They still cover ACA preventive care at $0 and at least three primary-care visits before the deductible. Eligibility is restricted: generally only people under age 30, or those who qualify for a hardship or affordability exemption, can buy one, and premium tax credits can’t be applied to them.