Learn in the textbookApplying Encryption and Digital Signatures
DefinitionFile encryption (ファイル暗号化) protects data by choosing files and encrypting them one at a time.
ExplanationAn encrypted file stays encrypted even when it is copied to other media or attached to an email. However, if the key or password for decryption travels over a route that could be snooped on just like the file, the contents are not protected. And any file you forget to select is left unprotected.
AnalogyIt is like keeping only your important papers in a small locked box. Even if someone is handed the whole box, they cannot open it without the key.