Serre duality is a fancy word that means that in math, there is a really cool trick to flip things around and make them look different, but still mean the same thing.
Imagine that you have a really big puzzle with lots of different pieces. The puzzle represents a complicated concept that you are trying to understand. You have some information about each piece, but it's hard to see how they all fit together.
Then someone tells you about Serre duality. This means that you can take each piece of the puzzle and flip it over, like looking at the back of a picture. When you do this, you can see a different side that you couldn't see before. But even though it looks different, it still fits into the puzzle in the same way.
So by using Serre duality, you can look at each piece of the puzzle from a different angle and understand it better. This is especially helpful when the concept is very complicated and hard to understand.
In math, Serre duality is used in a similar way to help understand complicated mathematical concepts called sheaves. Serre duality helps mathematicians flip things around and see the problem differently, which can make it easier to solve.