Granary is a control point map with 5 control points in total and an Arena map. To win a round a team must capture all 5 of these points, each team starts with the 2 control points closest to their spawn and the middle point is neutral. Once a control point is captured, the team's control point before it will be locked meaning the enemy team will not be able to capture it. Granary is very similar to Well.
The Arena version of the map was released alongside the Heavy update, the last two checkpoints on each side of the middle checkpoint were removed so that the map was the correct size for Arena mode. All that remains is the middle checkpoint and the two spawns either side of it.