Think of your favorite ice cream cone. When you start eating it, the ice cream is big and tall. But as you take more bites, the ice cream gets smaller and shorter. This is what we call a decrease. It means something is getting less or smaller over time.
Imagine a balloon filled with air. When it is full, it is big and round. If you let some air out, the balloon starts to shrink and become smaller. This is another way to understand decrease. The air inside the balloon is becoming less.
Picture a pile of building blocks. At first, the pile is tall with many blocks. But as you take blocks away, the pile becomes shorter and has fewer blocks. This is a decrease because the number of blocks is getting less.
Think about a sunny day when the sun sets. At noon, the sun is high and bright. As the day goes on, the sun gets lower and the light becomes dimmer. This is a decrease in sunlight as the day turns into night.