The American Health Care Act (AHCA) is Republicans’ plan to repeal and replace Obamacare.
Things that would stay: a ban on preexisting conditions, allowing young adults to stay on their parents’ plan up to age 26, and banning lifetime limits in health insurance.
Things that would change: Subsidy amounts, minimum standards of coverage (ie higher deductibles/copays will be allowed), a Medicaid reduction, changing the penalty for non-coverage.
The subsidy amount (the discount you receive to purchase healthcare) will be determined not by your income, but by age. Younger people can expect to benefit from this, but it will likely make coverage for older people much more expensive.
It would also eliminate the mandate that all Americans purchase health coverage or pay a fine, instead opting for a 1-year 30% increase on premiums if your coverage lapses as any time.
The Medicaid expansion would be also be phased out by 2020, meaning that no new applicants that would currently qualify could be added.
In summary: You won't face a penalty for not having coverage, but you would be subject to increase when you do need coverage. Your premiums will likely go up under this plan, and it is possible you could be paying more for skimpier coverage. Luckily, coverage for pre-existing conditions as well as preventative care coverage remain untouched. Younger people may have it better under the new age-rating, but older Americans may face steep premium increases. We don't know for sure if and or when this bill will pass, so it's best to assume there won't be ANY changes to your 2018 coverage.