The cellar is primarily accessed through your house. It is used for storage, brewing and winemaking. From here you can access tunnels the morgue, the church, the dungeon, the village and the alchemy lab after you removed the blockages.
At the beginning it contains some broken barrels and sacks, that contain precious material, which helps you in the beginning. Removing those is also important to make space for new workstations to produce
a mug of beer,
Bottle of apple ferment and
Bottle of berry ferment. The chest there contains an interesting cooking recipe. Use it to learn how to prepare simple and tasty food.
A section of the cellar is called the "barn cellar". This is where the chute of the crate factory ends, and the
crates are stored automatically on pallets. There's also a porter station available, where you can put a zombie at work to haul the crates over to the trade office.