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, upgrades, and various rewards, making it a valuable tool for frequent travelers. The program offers different tiers of membership, including Premier Silver, Premier Gold, Premier Platinum, and Premier 1K, each providing additional benefits like priority boarding and bonus miles. Members can also earn miles through partnerships with hotels, car rental companies, and other travel services, enhancing their earning potential.