Nobody at TurboTax or in this public forum has access to your tax return. So it's just not possible to give you a useful response.
If you have self-employment income over $400, then in addition to the "regular" income tax, you would also pay an additional 15.3% self-employment tax.
Different types of income (such as from gambling winnings and different types of investments) are taxed differently too.
It could also be that your employer did not process your W-4 correctly, or at all.
