Decreasing the number of uninsured people was a key goal of Obamacare. It clearly succeeded. The reduction in the uninsured has become a central defense of Obamacare as the GOP says it will repeal and replace it.
In 2013, the year before the major Obamacare provisions went into effect, there were more than 41 million uninsured. By 2015, this was reduced to 29 million. The uninsured rate, 9.1%, was the lowest in decades. But the problem hasn't gone away -- there are still almost 30 million uninsured people in the United States. Almost half of the remaining uninsured say they can't afford coverage. Many of these people live in states that didn't expand Medicaid, and don't qualify for financial assistance. Most of the uninsured are low-income.
Why this matters:
Republicans who want to repeal and replace Obamacare are probably going to be held to its coverage levels, and will be heavily criticized if they increase the number of people without coverage. Democrats are also highly unlikely to vote for any replacement that doesn't insure as many people.