We have great resources to help people understand tax brackets.
When determining a person’s taxes, there are a couple of different values that are used. These include taxable income, adjusted gross income (AGI), and modified adjusted gross income (MAGI.)
The Money Guy 2021 Tax Guide is an outstanding resource put together by The Money Guy team and updated annually. As noted in the tax guide, your marginal tax bracket is determined by your “taxable income.” Certain tax-related calculations are dependent upon what the AGI or the MAGI. Check The Money Guy Resources page for each year's guide.
Taxable Income is an excellent review article from Investopedia that explains how taxable income is determined.
For more information on smart tax planning, check out the following videos.