Not everyone is required to file an income tax return each year. Generally, if your total income for the year doesn't exceed the standard deduction plus one exemption and you aren't a dependent to another taxpayer, then you don't need to file a federal tax return. The amount of income that you can earn before you are required to file a tax return also depends on the type of income, your age and your filing status.
All Taxpayers are eligible to claim a standard deduction, and if not the dependent of another taxpayer, then one exemption as well. The standard tax deduction and exemption amounts are fixed by the government before the tax filing season and generally increase for inflation each year.
Since your income that is equal to or less than the sum of the exemption and standard deduction is not taxable, the IRS doesn't require you to file a return in years your income doesn't exceed that sum. When determining whether you need to file a return, you don't include tax-exempt income. In 2017 for example, if you are under age 65 and single, you must file a tax return if you earn $10,400 or more, which is the sum of the 2017 standard deduction for a single taxpayer plus one exemption.