Imagine you have a bag of toys and you want to measure how big it is. You could count the number of toys, but what if some toys are bigger than others? Then counting won't give you an accurate measurement.
The Gowers norm is like a special ruler that helps you measure how big the bag of toys is, taking into account how big each toy is. It's called a norm because it tells you how much "distance" there is between things.
To use the Gowers norm on your bag of toys, you would take each toy out and compare it to all the other toys. For every pair of toys, you would look at how different they are. The more different they are, the farther apart they are on your Gowers norm scale.
You would do this for all the pairs of toys, and then add up all the distances to get a final measurement of how big the bag of toys is. This measurement is called the Gowers norm of the bag of toys.
The Gowers norm is used in math to measure how different functions are. Just like some toys are bigger than others, some functions can be more complex than others. The Gowers norm helps us compare them accurately.