Medium is required by law to report royalty earnings over $0.50 to the IRS. The 1042-S is sent to nonresident aliens each January and summarizes the earnings paid and any taxes withheld during the previous calendar year.
In most cases, you do not need to take any further action for US taxes. You can use the form when filing taxes in your country of residence.
For a full explanation of how withholding rates are determined, see Earnings withholding for US taxes.
Medium is not able to provide tax advice. For guidance on filing your taxes, please consult a tax professional.