The known-plaintext attack is a cryptanalytic attack in which the attacker has samples of both the plaintext and its encrypted version (ciphertext) and is at liberty to make use of them to reveal further secret information; typically this is the secret key.
At Bletchley Park in World War II, strenuous efforts were made to use, and even force the Germans to produce messages with, known plaintext. The known plaintexts were called "cribs", and schemes to force the Germans to produce them were called "gardening".
Encrypted file archives such as ZIP are also very prone to this attack. For example, an attacker with an encrypted ZIP file needs only one unencrypted file from the archive which forms the "known-plaintext". Then using some publicly available software they can instantly calculate the key required to decrypt the entire archive.
To obtain this unencrypted file the attacker could search the website for a suitable file, find it from another archive they can open, or manually try to reconstruct a plain text file armed with the knowledge of the filename from the encrypted archive.
See also