Pantsers* write a story with only a general thought of how their story should go. On the opposite side are plotters who outline their story, sometimes in detail. I think most writers are half way between these two and use both techniques to craft their story based on how they write.
When starting off, most writers have been thinking about what they will write for some time. They have at least thought up the primary characters and a plot. It’s at this stage, before writing, they are a pantser and a plotter at the same time.
For a pantser to begin writing their story, they need to structure it at least in their mind. For a plotter to outline their story, they need to discover it by writing out the scenes. Therefore, writers are both a pantser and a plotter. It’s how they apply these techniques that makes the difference.
A pantser will have something in their head to follow. A plotter will have something written to follow. They both follow a plan to tell their story.
I’m a half way writer. I have it in my head how I want the story to go and I write a few pages. Then, I write the ending. That way I know where I’m going, even if I end up somewhere else. When finished, I summarize each chapter in a kind of outline. This is so I can remember who did what before I start my edits.
Really, in the end it only matters that the story gets written.
* Some pantzers prefer to be called discoverers.
P.S. My book High School Rocket Science (For Extraterrestrial Use Only) is now self-published as an ebook and in print. Yeah!