Commitment is one of the five scrum values. Collective commitment is a related to it. Remember what commitment says,
People personally commit to achieving the goals of the Scrum team.
This essentially means that each team member feels that his or her personal success is tied to the success of the project. Each team member also feels committed to each item in the sprint and not just the items they are working on. This is precisely collective commitment
Check more articles on Agile