MIRV stands for Multiple Independently targetable Re-entry Vehicles, and the US acquired such technology in the early 1970s, escalating the arms race to a dangerous new phase.
Essentially, this means you can place multiple warheads on one delivery vehicle or ICBM, and hit an individual target with each individual warhead. This is cheaper, for one thing, as you only need one missile for up to 12 warheads, and each nuclear weapon packs more punch in this way. Since there are fewer of them necessary in order to deploy the same yield as ordinary ICBMs, they are also easier to conceal and move.
Perhaps the greatest "value" to this technology is that you can place so many more warheads on nuclear missile submarines, which are much more mobile and more difficult to detect. This means that your nuclear arsenal is hidden most of the time, and therefore is impervious to attack. With the concept of Mutual Assured Destruction, a nuclear power is even less likely to strike against a country with MIRV technology.