To cut out, white-out or black-out parts of a document.
In legal proceedings, this is generally justified for reasons of privilege.
Although relevant documents have to be disclosed between litigants, some documents, in whole or in part, may contains references, parts or elements which are not subject to disclosure.
An example might be a long, relevant document which has a few paragraphs which contains a summary of legal advice protected by the client-solicitor privilege, jeopardize state security or reveal the identity of a state informer. If practicable, the document should be disclosed but "redacted for privilege"; with the confidential portion blacked-out or whited-out or otherwise removed.
Other examples of information that might properly be redacted from a document prior to disclosure to the other side in a law suit, include trade secrets, confidential and non-relevant medical or psychological information.
Generally, the Courts prefer a party to redact segments of a document for privilege, as opposed to the complete nondisclosure of a document, as it fosters full disclosure.