Medicaid expansion is one piece of the Affordable Care Act. The goal is for more people to be able to use Medicaid to pay their health care costs. In the past, only low-income people in certain groups, such as pregnant women, could get Medicaid. In 2014, anyone whose income is below or just above the poverty level and who lives in a state that has opted to expand Medicaid will qualify for benefits. This includes adults who don't have children. The act also made the minimum income requirement higher so more people can qualify.
The federal government will help states pay for the increase in people. States will get money for the cost of covering people who can use Medicaid for the first time.
Each state chooses whether or not they want to expand Medicaid as described in the Affordable Care Act. Some states have chosen not to accept the expansion. Those states will have lower income requirements for people to apply to Medicaid, so fewer people will be able to use it.