The Affordable Care Act (ACA) requires certain employers with at least 50 full-time employees (or equivalents) to offer health insurance coverage to their full-time employees/equivalents (and their dependents) that meets certain minimum standards set by the ACA. If these employers do not, they are required to make a tax payment called the employer shared responsibility payment (ESRP). The ESRP is also known as the employer mandate.

References

https://www.healthcare.gov/glossary/employer-shared-responsibility-payment/