A cross section in fiber means taking a look at what the fiber looks like when we cut it in half and look at it from the side. It's kind of like when you slice a piece of cake and can see all the layers. When we do this with fiber, we can learn about its shape and size, which can help us understand how it works and what it’s made of. It's like looking at a map of a house to see where all the rooms are and how they connect. By looking at the cross section of a fiber, we can see how it's put together and how it might behave in different situations.