On colonised planets, tiles are the spaces available to build planetary improvements. A planet starts with a number of tiles determined by its quality, and more can be gained by researching terraforming technologies and building the special improvements that they provide.
Tiles with a green outline are empty and can be built on immediately. Tiles outlined in yellow require Soil Enhancement before they can be used, while those with an orange or red outline require Habitat Improvement or Terraforming respectively.