You don't need health insurance at the federal level as of 2019. But some states require coverage and impose penalties. California residents, for instance, face fines exceeding $2,000 for going uninsured.
The ACA initially required most Americans to obtain health insurance or pay a tax penalty. This "individual mandate" aimed to create a broader, healthier insurance pool and reduce overall health care costs. Congress actively enforced the individual mandate from 2014 to 2019.