The idea embodied by this phrase describes the general penumbras and emanations
in the culture
of a society
that involve to what extent civil liberties
are expressed and defended in that society.
A culture of civil liberties exists when individual freedoms are held as carrying greater weight than national security in the balancing test applied to guaranteed, via the Constitution, or natural rights.
In the United States such a culture did not exist for a very long time. In fact, it was not until the Warren Court over came its timidity of McCarthyism that civil liberties such as the right to privacy first came into existence. The idea was that First Amendment freedoms needed breathing space to be effective. The Court was very watchful of laws that were overbroad and that would deter people from exercising Constitutional freedoms for fear of prosecution.
From this idea was born the Chilling Effects Doctrine, and the idea that a defendant being prosecuted under an overbroad law could point out not just his or her individual liberties being infringed upon, but also the damaging effect that the law would deter others from doing legal acts that made dubious by the overbroad law.
The only reason that this argument remains powerful is because freedom of expression and an open marketplace of ideas are considered essential to what makes up the American democracy. Any law that made people fearful of expressing themselves or of entering the marketplace of ideas was thus damaging to American national identity.
The consideration of such rights as fundamental to the American way of life is only possible when a culture of civil liberties exists.
In times of crises, the culture of civil liberties receives special attention because it is during these periods that it is most vulnerable and likely to be tampered with. For this reason it is also during these times wherein a society should be most concerned with preserving such a culture.