The Individual Mandate
Everybody has to have health insurance, or pay a penalty. The penalty goes away after 2018. You will need to pay any penalty owed for 2018 when you file your taxes in 2019.
There is a federal government penalty for people who don't have health insurance. The penalty is in place to encourage people to have health insurance.
You will not be penalized if you have 'minimum essential coverage' as defined in the law. If you have health insurance through your employer, you won't have to pay the penalty. In fact, most types of health insurance will excuse you from the penalty, including plans from Medicare, Medicaid, CHIP, veteran's health program, the Indian Health Service, TRICARE, or a plan you buy yourself. Not all health plans offer minimum essential coverage. Short-term disability and many other limited medical plans such as accident, critical illness and indemnity do not offer minimum essential coverage. If you truly can't afford a plan, you may qualify for federal subsidy assistance from the government to help pay for one (See Affordability).
If you choose to pay the penalty rather than buy health insurance, you will have to pay for all your health care costs. Paying the penalty does not give anyone free health care or health insurance coverage.