Mass is like the amount of stuff inside something. Think of it like a big jar of candies. The more candies you have in the jar, the heavier it feels when you try to lift it. Just like that, when something has more mass, it means it has more stuff inside it and feels heavier.
Imagine you’re playing with building blocks. If you stack up a few blocks, it’s easy to carry. But if you stack a whole lot of them, it becomes harder to lift. That’s because the mass of the blocks has increased with each block you add. More blocks mean more mass.
Think about a balloon filled with air. It doesn’t weigh much because it doesn’t have a lot of mass. But if you fill it with water, it becomes much heavier. That’s because water has more mass than air, so the balloon is heavier when it’s filled with water.
Consider a feather and a rock. A feather is light because it doesn’t have much mass. A rock is heavy because it has a lot of mass. So, mass is what makes things feel heavy or light.