Actually both. It will reduce the Net Profit which you are taxed on and also reduce the separate self employment tax.
You will pay Self Employment tax (Scheduled SE) on a Net Profit of $400 or more on Schedule C in addition to regular income tax on it. You pay 15.3% SE tax on 92.35% of your Net Profit (If it is greater than $400). The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare. So you get social security credit for it when you retire.
