The term forest in the ordinary modern understanding refers to an area of wooded land; however, the original medieval sense was closer to the modern idea of a "preserve" – i.e. land legally set aside for specific purposes [...] Royal forests usually included large areas of heath, grassland and wetland – anywhere that supported deer and other game. [...]
The areas that became Royal Forests were already relatively wild and sparsely populated, and can be related to specific geographic features that made them harder to work as farmland. Prosperous, well-farmed areas were not generally chosen to be afforested; if they were, they tended to lose the status fairly rapidly.