A secure storage container or room for valuables, documents, or important items. Can refer to a physical safe, a bank vault, or a locked cabinet.
Please keep important documents in the safe.
I deposit valuables in the bank's vault.
金庫 specifically implies a lockable safe for money or valuables, while 保管庫 is a more general storage vault that need not be lockable.
保管庫 stresses secure, controlled storage like a vault or cabinet, while 倉庫 simply means a warehouse building.
Compound of 保管 (ほかん, 'storage, safekeeping') and 庫 (こ, 'warehouse, storehouse').