Duck and Cover was a method of personal protection against the effects of a nuclear explosion, which the United States government taught to generations of United States school children from the early 1950s until the end of the Cold War in the late 1980s.
It was intended to protect us the event of a nuclear attack, which, we were told, might come at any time without warning.
Should there be a surprise attack, immediately after we saw a flash we were to get on the ground under some cover—such as a school desk, or at least next to a wall—and assume a prone position, lying face-down and covering our exposed skin.
Throughout our years as a primary and secondary-school student, we were forced to practice these air-raid drills.
Here is a link to a remarkable, and frightening cartoon that was created at that time: