What did Carl Jung mean by collective unconsciousness?

The Collective Unconscious is term and concept popularized by psychologist Carl Jung, considered a part of analytical psychology. Jung called it "a reservoir of the experiences of our species."

In other Words, each person has an individual "psychology" and also a group or shared "psychology" that is made up of the combined shared experiences of humans. This encompasses everything from science to religion and philosophy. Jung believed that individual psychology influences the individual while the collective unconscious influences the whole group. The influence may not be on a level perceived by the group, but it is non the less a real force in his view.

You can read more about these views in Jung's work Psychological Types.