Research improves all aspects of your kingdom, whether that is in food production, or the speed of your armies, the capacities of your granary, or the speed at which your buildings get built. There are over 700 researches to be carried out across over 80 categories. The order you choose to do them reflects your style of play. Some researches allow access to other categories, a fantastic way to understand this is to click on the ‘tech tree’ tab in the research screen.