Refers to a floor below ground level in a building, used for storage, parking, or commercial space.
There is a parking lot in the basement of this building.
We keep the inventory in the basement storage room.
一階 is the street-level floor of a building, while 地階 is a basement level below the ground.
Compound of 地 (chi, 'ground') and 階 (kai, 'floor'), literally 'ground floor' but used to mean 'basement'.