The Affordable Care Act requires nearly everyone to have health insurance that meets minimum standards. With some exceptions, people who do not maintain ACA-compliant health insurance coverage pay a penalty.
The GOP tax bill that was enacted in late 2017 will eventually repeal the individual mandate penalty, but that won’t take effect until 2019. People who are uninsured in 2018 will still face a penalty (the same penalty that applied for 2017), but people who are uninsured in 2019 will not be subject to a penalty. The penalty is collected on tax returns, so people who are uninsured in 2018 will pay their penalty in 2019.