The Granary is a building.
upgrade => Great Granary
This building requires 3000 man-days
The construction of the building requires 1 political point, 4,000 gold and 30 wood
The granary allows you to store more food (150 instead of 50).
It also allows you to distribute food to the people, in order to increase the birth rate of the province three times a year (+0.2%)