In Minecraft, chunk borders are invisible boundaries that divide the world into 16×16 block sections. By default, these borders are not visible in-game, but there is a way to turn them on.
There are a few reasons why you might want to turn on chunk borders. First, they can be helpful for debugging world generation issues. Second, they can be used to create interesting visual effects. Third, they can be used to help you navigate the world, especially if you are using a minimap mod.