What is NATO? Why NATO is important to the United States?
NATO stands for the North Atlantic Treaty Organisation, and was created in 1949 after the Berlin Blockade failed.
NATO is important to the US because it helped to secure Europe and North America during the Cold War, it was an alliance based on the concept that an attack on one, is an attack on all which meant if one nation was attack (presumably by the USSR) then it'd count as an attack on all the members and war would occur.
The US saw it as a defensive alliance during the Cold War, but the USSR/Stalin saw it as an aggressive alliance against the USSR, and was frightened by the thought of war. Especially the US due to them having the atomic bomb.
The Soviet Union in 1955 created the Warsaw Pact, where all of the countries behind the Iron Curtain joined, and was in direct response to West Germany joining that year due to his paranoia at an 'enemy on the doorstep'.