The filing threshold is the minimum amount of income you must earn in order to be required to file a federal income tax return. The amount varies depending on whether you’re under 65, your marital status, and whether you have dependent children (ie, your filing status: single, head of household, married filing jointly, married filing separately, or qualifying widow(er) with dependent children).
The IRS has a page that shows tax filing thresholds for each filing status, for people under 65 as well as those 65 and older.
In the context of the ACA, the tax filing threshold comes into play when we talk about the individual mandate penalty. There’s an automatic exemption from the penalty if your income is below the tax filing threshold. And if the percentage of income penalty applies to your tax return, it’s only calculated on the portion of your income that exceeds the tax filing threshold for your situation.