Wages that are reported to you on Form W-2 have already been subjected to FICA (Social Security and Medicare) tax withholding.
Self-employment income, reported on Form 1099-MISC, Box 7, has not been subject to FICA tax before you receive it. When you complete Schedule C and determine your net self-employment income, that income will be subject to self-employment tax, which is both the employee's share of FICA and the matching employer's share of FICA. That is to say that net self-employment income, if you have more than $400 in any year, will be taxed at 15.3% for FICA in addition to income tax.
