When you bring a foreign driver license to a U.S. DMV, the clerk needs to verify your driving history — name, date of birth, license class, issue date, expiration date, and any restrictions. If this information is not in English, the clerk cannot process your application. A certified English translation resolves this by providing an official English version of every element on your license.
Most states accept a "certified translation" — meaning a translation performed by a professional translator with a signed statement certifying accuracy and completeness. Some states go further. Washington DC requires the translation on embassy letterhead or from a certified translation company with a translation stamp or notarized affidavit. Maryland requires an MVA-approved interpreter/translator. New Mexico requires the translator to certify the translation as "full and complete."
The key distinction is between visiting and converting. If you are visiting the U.S. on a tourist visa or short-term stay, you can usually drive on your valid foreign license for a limited period. An IDP or certified translation helps if you are stopped by police or need to show identification. But if you are becoming a resident, every state requires you to obtain a local driver license within a specified period — and a certified translation of your foreign license is part of that process.