Simple question. If Person A makes $150,000 at their regular job (W2) and brings in $5,000 on a side job (1099-MISC), do they owe more in taxes on the $5,000 versus Person B, who earns $50,000 (W2) at their regular job?
US income tax works this way: it is a marginal tax, i.e. the tax is calculated from each additional dollar of income, according to the following rule: there's a number of "brackets", and once you income is in certain bracket, the additional dollars are taxed according to this bracket, until you have enough income to go to higher bracket. Current brackets are, for single person:
10% $0 – $8,375 15% $8,376 – $34,000 25% $34,001 – $82,400 28% $82,401 – $171,850 33% $171,851 – $373,650 35% $373,651+
This means that if you earned $150000, you are in 28% bracket, and so you'll pay 28% income tax, or 1400, on additional 5000 of income. If you earned only $50000, you would be in 25% bracket, so you would pay only 1250.
In fact, it's more complicated since you can take various deductions before your income is considered for the purpose of taxation, and it can take you to different tax bracket sometimes, if you are close to the margin, so if you want to know the real tax, you better use some specialized software or talk to an accountant. But for the purpose of quick explanation, that's how it works.