The new directive applies to returning U.S. citizens as well as foreign travelers.
The order becomes effective on January 26, 2021.
The order covers nine pages of detailed explanation, but the gist of the directive is summarized on the CDC website with this message:
To reduce introduction and spread of new variants of SARS-CoV-2, CDC issued an Order
effective January 26, 2021. It requires all air passengers arriving to the US from a foreign country to get tested for COVID-19 infection no more than 3 days before their flight departs and to provide proof of the negative result or documentation of having recovered from COVID-19 to the airline before boarding the flight.