Editorial
Without prejudice
Definition
A label on communications made in a genuine attempt to settle a dispute, which are generally inadmissible as evidence of admissions. The protection depends on the substance of the communication, not on the words used.
In plain English: Things said while genuinely trying to settle usually cannot be quoted against you in court later.
You did not name a jurisdiction, so this shows the GB definition. The term is defined differently elsewhere — see below.
Usage
Writing 'without prejudice' on a letter that is not a settlement attempt does not make it privileged, and omitting it from one that is does not remove the protection.