An unconditional surrender is a surrender in which no guarantees, reassurances, or promises (i.e., conditions) are given to the surrendering party. It is often demanded with the threat of complete destruction, extermination or annihilation.
Announcing that only unconditional surrender is acceptable puts psychological pressure on a weaker adversary, but it may also prolong hostilities. A party typically only demands unconditional surrender when it has a significant advantage over their adversaries, when victory is thought to be inevitable.
In modern times, unconditional surrenders most often include guarantees provided by international law. In some cases, surrender is truly accepted unconditionally; while in other cases terms are offered and accepted, but forces are declared to be subject to "unconditional surrender" for symbolic purposes. This type of surrender may also be accepted by the surrendering party under the expectation of guarantees agreed to informally.