Short answer: EHBs are 10 categories of care that every ACA individual and small-group plan must cover, including hospitalization, prescription drugs, maternity, mental health, and preventive care.
The Affordable Care Act requires individual and small-group plans to cover ten Essential Health Benefit categories: ambulatory (outpatient) care, emergency services, hospitalization, maternity and newborn care, mental health and substance-use treatment, prescription drugs, rehabilitative services and devices, laboratory services, preventive/wellness and chronic-disease management, and pediatric services (including dental and vision). Each state picks a benchmark plan that defines the specifics, so details vary by state, but no compliant plan can leave out an entire category.