The masses refers to a large, general group of regular folks — the common people of a society. If you’re super rich, you ride around in a private jet while the masses take crowded buses.
The masses don’t include celebrities, rich people, royalty, or political leaders. The masses are everybody else. People use this term when they need to speak generally about the majority, although it often refers to the lower class. To win an election, a politician must appeal to the masses. Popular movies and music also must appeal to the masses — in other words, they need mass appeal. The word masses is also just plural for any kind of mass.