Short answer: It depends on your state and income. In states that expanded Medicaid, you qualify on income alone if your household earns up to about 138% of the federal poverty level. In states that did not expand, eligibility is narrower, and some low-income adults fall into a “coverage gap” where they qualify for neither Medicaid nor Marketplace subsidies.
The ACA gave states the option to extend eligibility to adults with income at or below 133% of the FPL; because of how income is counted, this works out to about 138% of the poverty level. In an expansion state, if your household income is below 133% of the federal poverty level (FPL), you qualify based on income alone, regardless of other factors.
If your state has not expanded Medicaid and your income is below the poverty level, you may not qualify for Medicaid under the state’s rules and also cannot get Marketplace premium tax credits, the so-called coverage gap. It is still worth applying, because you might qualify based on having children, pregnancy, or a disability, or for sliding-scale care at a community health center. If your expected income later rises into the 100% to 400% FPL range, you can become eligible for Marketplace subsidies.