Z-order is like stacking pancakes on top of each other. Imagine you have three pancakes, one with chocolate chips, one with blueberries, and one plain. You want to stack them on top of each other, but you want to be able to see all the pancakes.
We can do this by giving each pancake a number. The pancake with chocolate chips is number one, the pancake with blueberries is number two, and the plain pancake is number three.
Now, let's stack them up! We start with number one, then we put number two on top of it, and finally, we put number three on top of all of them. This way, we can still see all the pancakes, and we know which one is on top because of their numbers.
In computer programs, we use z-order to stack different things on top of each other. Just like the pancakes, each thing has a number, and we put them in order from the bottom to the top. This helps us see everything on the screen in the way we want to. For example, we can have a picture on top of a background with some text on top of the picture, and we can still see everything because of the z-order.