Your total income minus specific adjustments the tax code allows, before the standard or itemized deduction.
AGI is line 11 of Form 1040. It is the number the IRS uses for the estimated tax safe harbor (110 percent of prior-year tax applies above $150,000 AGI), for income-based phase-outs, and for the filing thresholds many states copy. It is not taxable income; that comes after deductions.