A more thorough explanation:

A deficiency notice, also known as a ninety-day letter, is a legal notice sent by the Internal Revenue Service (IRS) to a taxpayer informing them of a tax deficiency. This means that the IRS has determined that the taxpayer owes more taxes than they have paid. The notice gives the taxpayer 90 days to either pay the taxes owed or challenge the deficiency in tax court.

For example, if a taxpayer files their tax return and the IRS determines that they owe an additional $5,000 in taxes, the IRS will send a deficiency notice to the taxpayer. The taxpayer then has 90 days to either pay the $5,000 or challenge the deficiency in tax court.

It is important for taxpayers to respond to a deficiency notice within the 90-day period, as failure to do so can result in the IRS taking collection actions, such as garnishing wages or seizing assets.