When you work as a W-2 employee, your employer pays half of your Social Security and Medicare taxes (7.65%) and withholds the other half from your paycheck. When you're self-employed, there's no employer to split that cost, so you pay both halves yourself: 15.3% total, applied to 92.35% of your net self-employment earnings.
This is on top of, not instead of, regular federal and state income tax — which is why many new freelancers are caught off guard the first time they see their full tax bill. A common rule of thumb is to set aside 25-30% of net profit for taxes throughout the year, since the US tax system expects self-employed people to pay estimated taxes quarterly.
Half of the self-employment tax paid is deductible when calculating adjusted gross income, which softens the blow slightly, but it's still the single biggest tax-planning surprise for people transitioning from a salaried job to self-employment.