A multiple independently targetable reentry vehicle (MIRV) warhead is a collection of nuclear weapon

Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

s carried on a single intercontinental ballistic missile

Intercontinental ballistic missile

An intercontinental ballistic missile is a ballistic missile with a long range typically designed for nuclear weapons delivery...

 (ICBM) or a submarine-launched ballistic missile

Submarine-launched ballistic missile

A submarine-launched ballistic missile is a ballistic missile capable of delivering a nuclear warhead that can be launched from submarines. Modern variants usually deliver multiple independently targetable reentry vehicles each of which carries a warhead and allows a single launched missile to...

 (SLBM). Using a MIRV warhead, a single launched missile can strike several targets, or fewer targets redundantly. By contrast a unitary warhead is a single warhead on a single missile.
The military purpose of a MIRV is threefold:

• Provides greater target damage for a given missile
  Missile
  Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...
  
   payload.