A payment ("fee," "fine," "individual mandate") you make if you don’t have health insurance that counts as qualifying health coverage. The penalty in 2016 and 2017 for not having health coverage is $695 for each person on your tax return who isn't covered ($347.50 per child), or 2.5% of your household income, whichever is more. The fee rises with inflation. Final 2018 amounts will be published when available.
If you're uncovered just some months of the year, you pay 1/12 of the penalty for each month you're uninsured. If you're uncovered for only 1 or 2 consecutive months, you don't have to pay the fee at all.
People with very low incomes and others with special circumstances may be eligible for exemptions from the requirement to have health insurance. If you qualify for an exemption, you won't have to pay the fee.