Generally, you can only contribute to a Roth IRA if you have taxable compensation and income less than the top of the phaseout range for your filing status (see chart below). If your income is greater than that threshold amount, you are prohibited from contributing directly to a Roth IRA.
A “backdoor Roth IRA” allows a taxpayer to bypass income limitations by first making a nondeductible contribution to a traditional IRA and then converting it into a Roth IRA. If the conversion happens soon after the original contribution, there will be little or no taxable income or tax assessed. Due to the distribution rules for traditional IRAs, this works best if you have no other traditional IRAs.