In many disciplines a greenfield is a project that lacks any constraints imposed by prior work. The analogy is to that of construction on greenfield land where there is no need to remodel or demolish an existing structure. Such projects are often coveted by engineers.
A Greenfield Investment is the investment in a manufacturing, office, or other physical company-related structure or group of structures in an area where no previous facilities exist.   The name comes from the idea of building a facility literally on a "green" field, such as farmland or a forest. Over time the term has become more metaphoric.