The different moods of your people.
The Mood of your people must be carefully maintained. A happy populace is productive, and is likely to have breakthroughs and surges of productivity. An unhappy populace may be less productive, and in extreme circumstance can revolt or riot. The Mood of your people is determined by the actions preformed by you on your settlement. It can decrease if there is a negative food or gold production. It also decreases if you cancel trade links between villages. You can increase the mood by researching Religion or Philosophy, and by gathering Luxuries around the world.
Mood is a currently unreleased feature which means that all avaliable research projects that only 'Improve Mood' are completely pointless unless you need them to build a Wonder or to continue to the next section of a research progression, even though no such situation currently exists.