server – Understanding the importance of Gunicorn and Nginx for Django web development

I’m entirely uninitiated to the world of web development, and only have a tentative grasp on Django and web development through the test server it works through.

From the guide I’m reading, the author turns to using Nginx once he starts working on site deployment, because Django is “not designed for real-life workloads.” What does that mean, and why doesn’t it? In terms of justification for using Gunicorn, the author remarks:

Do you know why the Django mascot is a pony? The story is that Django
comes with so many things you want: an ORM, all sorts of middleware,
the admin site…​ “What else do you want, a pony?” Well, Gunicorn
stands for “Green Unicorn”, which I guess is what you’d want next if
you already had a pony…​

Well and good, but I don’t really know what the two are doing for the server. I know for web developers this is like asking what multiplication is to a maths professor, so please excuse the naivety. In your please keep in mind I have almost no knowledge of web development other than what I’ve thus far learned from this guide, doing my best to understand as much as I can for the previously entirely uninitiated (I’m from a computational programming background).