NATO, the North Atlantic Treaty Organization, was founded in 1949. It is a mutual defense league. When an attack is made on one member, all of the members are automatically at war with the attacker.
Originally, the US and Canada are members, along with 10 other nations. Together they formed an alliance as protection against the USSR. With all of them united, it would be harder for a country to attack all of them at once.
This event was important to the Cold War because it created a group of nations led by the US who were all united against the USSR, but again, without any direct fighting.