The Affordable Care Act (ACA), also known as Obamacare, was signed into law by President Barack Obama on March 23, 2010. The law sought to overhaul the U.S. healthcare system by introducing several provisions aimed at making healthcare more accessible and affordable, in response to the continually rising costs of healthcare in the country.

At its core, the ACA attempted to decrease the number of uninsured Americans and reduce the overall costs of healthcare in several ways:

1. Expansion of Medicaid: The ACA provided states with the option to expand their Medicaid programs to cover all adults with income up to 138% of the federal poverty level. This expansion was significant because Medicaid typically provides health coverage for some low-income people, families and children, pregnant women, the elderly, and people with disabilities.

2. Health Insurance Marketplaces: The ACA created health insurance exchanges or marketplaces where individuals, families, and small businesses could shop for and purchase health insurance coverage. These marketplaces were designed to foster competition among insurance providers, thus helping to lower premiums.

3. Premium Tax Credits and Cost-Sharing Reductions: To make health insurance more affordable, the ACA introduced subsidies like the Premium Tax Credit, which lowers the cost of health insurance for eligible individuals and families purchasing coverage through the health insurance marketplace. Cost-sharing reductions were also offered to lower out-of-pocket costs for lower-income individuals.

4. Individual Mandate: The ACA required most Americans to maintain “minimum essential” health insurance coverage; if not, they would face a penalty. This mandate was intended to broaden the health insurance risk pool and, in theory, lower costs for everyone by ensuring that healthier individuals, who are less likely to use healthcare services, would also have insurance coverage.

5. Employer Mandate: The ACA also required larger employers (those with 50 or more full-time employees) to provide affordable health insurance coverage to their employees.

In terms of effectiveness, several studies have shown that the ACA has made a substantial impact on reducing the number of uninsured individuals. The ACA has also been associated with a slowdown in the growth of healthcare costs.

It’s worth noting that, while the ACA has made significant strides in addressing rising healthcare costs, it’s not a cure-all. Other issues, like high drug costs and opaque pricing practices within the healthcare 

industry, still contribute to high overall healthcare costs. Nevertheless, the Affordable Care Act represents a substantial legislative effort to combat the issue of rising healthcare costs and increase access to healthcare coverage in the United States.