From the horses mouth:
The term “household income” means, with respect to any taxpayer for any taxable year, an amount equal to the sum of—
(ii) the aggregate modified adjusted gross incomes of all other individuals who—
(II) were required to file a return of tax imposed by section 1 for the taxable year.
Modified adjusted gross = your AGI +non taxable Social Security Benefits + tax exempt interest + foreign earned income & housing expenses .
That said, for most taxpayers “household income” will basically be the AGI of you and your dependents that are covered under your health plan who file income tax returns.