Definition:
An Improvement Backlog lists everything that the organization could do better in its use of Scrum. Scrum development team uses product backlogs, team can use an improvement backlog to track the effort of adopting Scrum in organization.
To list out few Items that can make to improvement backlogs are increase adoption of test automation, enable teams to implement continuous integration, figure out how to make sure each team has a product owner, determine how team measures the impact of adopting Scrum. Improvement backlog will emphasize creation of awareness and desire, developing the ability, promote success or transfer the knowledge to other groups.
Further Reading:
Book: SUCCEEDING WITH AGILE Software Development Using Scrum by Mike Cohn