It’s a phrase that has recently become popular, although it is, by no means, new. It goes back to 1955, when the Quakers used it to urge our country to stand up to all forms of totalitarianism, especially fascism. (It was the title of one of their pamphlets.)
Today, it refers to holding a deep belief in what you say about injustice and using all nonviolent means to have people hear you. It’s standing up and speaking up, often (but not always) to government. “Speak truth to power” takes the courage of your convictions.
Too often the phrase is used to justify saying something to Authority that people in authority don’t want to hear.
But, for many, it’s intention is the same today as it was in the 1950s: to change our old thinking about violence and injustice; to reject both; and to embrace peace and justice. I like that idea very much.