Form 1042-S is used to report amounts paid to foreign persons that are subject to income tax withholding, even if no amount is deducted and withheld from the payment because of a treaty or exception to taxation, or if any amount withheld was repaid to the payee. It is also used to report amounts withheld under Chapter 3 of the Internal Revenue Code.
Sometimes, this form is mailed out strictly for informational purposes and therefore does not need to be included on your return. This is often the case when Resident Aliens receive this form.
The significant items on the 1042-S form are Box 7 and Box 10. Box 7 is US Federal Tax Withheld and Box 10 is Withholding Credit. If both of those boxes are 0, then nothing was withheld and there is no withholding credit and the 1042-S form is just for your informational purposes.
Complete instructions for filing your income reported to you on Form 1042-S, as well as descriptions of all other income codes reported on this form, can be found here: 1042-S instructions