United MileagePlus is the frequent flyer program of United Airlines, allowing members to earn miles for flights and other travel-related activities. Members can redeem these miles for free flights, seat upgrades, and other rewards, making it a valuable tool for frequent travelers. The program offers various membership tiers, including General, Silver, Gold, Platinum, and 1K, each providing different benefits such as priority boarding and bonus miles. Members can also earn miles through partnerships with hotels, car rental companies, and credit card providers, enhancing their earning potential and travel experiences.