Stephen Colbert was taking about the CIA Interrogation Report when he said, "unfreeze your heart!" @6:12 in the video.
What does that mean? How can I use that term? Does it mean, 'forget about it"?
"Unfreeze your heart" is actually a phrase from a popular song, and means to stop having a cold heart, i.e. start caring. A cold-hearted person is a person who doesn't care about people, and does not respond to their suffering. The opposite terminology is to have a warm heart, or be a warm-hearted person. Such a person reacts to others' suffering by either offering genuine sympathy or, if possible, relief.
In common speech it would probably be a bit pretentious to say "unfreeze your heart!" As it turns out, Colbert is a pretentious person and an entertainer, so it's OK for him.
A "heart of ice" is a heart that does not care, because it is cold. "Unfreeze your heart" means to stop being cold and uncaring. (Note: this is all based on the traditional idea of the heart as the source of feelings).