A certified translation is a word-for-word translation of an official document, accompanied by a signed Certificate of Accuracy. The certificate is a legal declaration that the translation is complete and accurate, signed by the translator or an authorized officer of the translation company.
In the United States, USCIS requires that any foreign-language document submitted as evidence must be accompanied by a certified translation per 8 CFR 103.2(b)(3).
It is not a notarized translation (that's a separate add-on). It is not a machine translation with a stamp. And without it, your application can be rejected, delayed, or returned — causing weeks or months of additional processing time.