The term “greenfield” comes from physical development- a you are taking a “green field” and turning it into a factory or housing or similar. It’s contrasted to “brownfield” which means developing in or over an already built up area.

In software development “greenfield” generally refers to new systems development whereas “brownfield” refers to the maintenance, extension, or enhancement of existing systems and solutions.

Greenfield development is a way of saying "new and from scratch" usually, whereas editing an existing codebase would not be.

Sibling comments both answered the greenfield thing as I would have :)

As for first business, I built a very small network over 7 years working in house at an advertising agency (which did a lot of digital work) and some startups. While I was just an IC developer, because I could communicate with non-technical people well, I became the first point of contact for a lot of decision makers over time.

If you want to do solo technical consulting start making friends with non-technical people who are in leadership roles. It takes time but it does snowball.