In short, copywriting is really moving words around to make them sound better. It's not just about "writing," it's about psychologically influencing people to do something, whether that's to purchase something or sign up for your email newsletter.
The best way to get started on understanding copywriting is the read the best of the best books.
Here are my favorites:
1. This Book Will Teach You to Write Better - Neville Medhora
2. The Copywriter's Handbook - Robert W. Bly
3. Ogilvy on Advertising - David Ogilvy
4. The Boron Letters - Gary C. Halbert
5. Scientific Advertising - Claude C. Hopkins
6. Advertising Secrets of the Written Word - Joseph Sugarman
7. The War of Art - Steven Pressfield
Tell me: what are your favorite books to read in your trade? What books do you think every professional should read?