A backlog is an organized, prioritized list of Features and Enablers. The Program Backlog 1 is continuously refined and reprioritized by the Product Management 2.
References
1 - Scaled Agile Framework - Program Backlog https://www.scaledagileframework.com/program-and-solution-backlogs/2 - CCSQ Agile Playbook - Agile Roles https://qnetconfluence.cms.gov/display/LACE/Agile+Roles