Greenfield land is land on the edge of a town/city, or away from larger urban areas. Greenfield land is undeveloped but has development proposed for it. These factors contribute to greenfield land being bought for speculative land investment, as planning may be more likely in these areas.
The development of greenfield land is certain to lose public favour. Councils are against the building of new houses on greenfield - tens of thousands of houses are built on greenfield land each year!