The Zoom window lets you write very neatly with your large finger or stylus. The window at the bottom magnifies a part of the page which corresponds to the blue box on the page.
Writing with auto-advance
For a fluent note-taking experience, GoodNotes lets you write continuously in the Zoom window. As soon as you cross the middle of the Zoom window with any pen stroke, a blue area will appear both on the left side of the window and on the page. Continue writing in this area will automatically move the blue box on the page forward. Auto-advance can be disabled as instructed below.
Additional settings of the Zoom window
To enable/disable auto-advance: From the document view, open the More menu (...) in the top-right corner > Document Editing, and toggle the "Auto Advance" setting.
You can adjust the Zoom window return height for a specific template. See: How to adjust Zoom window return height for a template