In the Solidity docs section on memory, you find this paragraph (emphasis added):
Each account has a data area called storage, which is persistent between function calls and transactions. Storage is a key-value store that maps 256-bit words to 256-bit words. It is not possible to enumerate storage from within a contract and it is comparatively costly to read, and even more to modify storage. A contract can neither read nor write to any storage apart from its own.
What is meant by the part in bold?