Marginal rates apply to income step by step
Tax brackets work like water filling successive containers. The first $11,000 of income for a single filer is taxed at 10%, the next chunk up to $44,000 at 12%, and so on. Only the income within each bracket faces that bracket's rate, not the entire income.
This is why jumping to a higher tax bracket never results in lower take-home pay. The higher rate applies only to the marginal income entering that bracket, not retroactively to all income.
Effective rate versus marginal rate
The marginal rate is the tax rate on the next dollar earned. The effective rate is total tax divided by total income. Someone with a 24% marginal rate might have a 16% effective rate because lower-bracket income was taxed at lower rates.
Tax planning often focuses on marginal rate management: timing large income events or charitable deductions to stay in a lower bracket, or recognizing that earning a bonus might push income into a higher bracket and cost more in taxes than the bonus amount.