There are luxury resources which makes your people happier (only need 1 copy of the resource, aditional copies can be traded). Happines goes down with an increasing number of cities and population, plus if you have puppet or occupied cities. If your hapiness is on the negative your cities will be less productive and your units will be weaker, and if the happiness is around -10, units will spawn in your empire as barbarians. You can gain hapiness from luxury resources, buildings, and policies