Special resources are favorable conditions that may be found on certain tiles on a planet. Building a planetary improvement on a special resource will see that improvement receive a bonus based on the type of resource it is. Note that the bonus increases the capacity of the improvement, not necessarily its output. For example, a manufacturing improvement built on Rare Elements (+300% industry) will also require +300% BCs to be spent. So you should NOT think of these bonuses as "buy one improvment, get 1 (or 3 or 7) free".