Windows in Vim represent a view of an underlying buffer. There is nothing stopping two different windows having the same underlying buffer though, which allows you to have two or more different views of the same file that you’re working on.
Here’s a situation where this is useful: imagine you’re working in a large file, and you’re writing some new text at the bottom, but you want to keep the content at the top visible for reference. If you’re currently at the bottom of the document, you can create a new split (horizontal with :sp or vertical with :vsp), and scroll to the top of the document with gg. When you move back to your initial window, you can continue your work as before, this time with top of the file easily visible.
Try this out in the editor! If you’re not familiar with windows, don’t worry — we’ve got a lesson coming soon, so check out the homepage. For now, you should know that :sp can be used to create a new horizontal split, :vsp creates a new vertical split, and :wincmd w cycles through the open windows.