Tile improvements are buildings or installations constructed outside of cities. Each improvement changes the properties of the tile where it's built, usually adding potential to it. Improvements are unlocked via certain technologies. Also, there are some improvements only available to certain civilizations. And finally, there are certain improvements that can only be built by using Great People. Improvements built on certain strategic and luxury resources are required for an empire to access the resources' special properties. This is the list of improvements in Civilization V.