Metros' are really big cities with lots of buildings and people all crammed together.
Metropolis is a fancy word people use to describe a really, really big city with lots and lots of people who live and work there. It's kind of like a giant beehive or ant farm, with lots of streets and buildings all packed closely together. You might see tall skyscrapers shooting way up into the sky, or bustling sidewalks packed with people rushing around to get where they're going.
At first, "metropolis" might sound like a really hard word to say or understand, but it just means a really big city where lots of people live, work, and make their home. Whether it's New York, Tokyo, or Mumbai, metropolises are exciting places full of sights, sounds, and experiences that you'll never forget!