This is a military strategy of deterrence in which the usage of nuclear weapons would result in the complete annihilation of both countries.

Mutually assured destruction, or mutually assured destruction (MAD), is a doctrine of military strategy and national security policy in which a full-scale use of high-yield weapons of mass destruction by two or more opposing sides would cause the complete annihilation of both the attacker and the defender

Significance: The significance of having the MAD is that it helps any nation involved in several ways. The nation does not have to put so much money into the military budget (spending money to develop and research nuclear weapons). Many lives are saved due to the fact that there are restrictions on nuclear warfare, meaning the war isn't as bad as it normally would be. If nuclear weapons are used in a war, more than likely, the entire planet's population of humans will be killed off, including other animals as well. Limiting nuclear warfare is probably the most significant part of the Cold War because the USSR or USA could have wiped each other out (including the rest of the world), but by limiting nuclear warfare, it prevents World War III from breaking out.

I hope this helps!
Good day to you.
-Niko