Basically, since the IRS requires things to be rounded to the nearest dollar, if you earned 50 cents or less in the tax year, as far as the IRS is concerned your reportable income for the year was zero. So you don't even bother filing a return.
If you made anywhere between 51 cents and $6350 for the year, then you "can" file a tax return. but you don't have to. The only reason for filing a return in this case, would be to get any withheld taxes on that income refunded to you.
Now, if you are self-employed, if you earned more than $400 of self-employment income you would be required to file a return even if your total income for the year was less than $6350. That's because when one is self-employed, you have to pay the additional Self-employment tax on any self-employment income in excess of $400.So if you have $1000 of self-employment income for the year and that's your only income, you're required to file. While you will not pay "regular" tax on that $1000, you *will* pay self-employment tax on the business income that is in excess of $400 - which in my example means you would pay self-employment tax on the additional $600.
