Guam is a U.S. territory, so if you are traveling under the U.S. Visa Waiver Program, you need an approved ESTA before departure.
Whether ESTA for Guam is required can vary depending on your nationality and method of entry, but if your itinerary involves U.S. entry, it is important to check and prepare your travel authorization in advance.
Since it may be checked before boarding or during entry procedures, please apply for ESTA before your departure for a smoother trip.