A backlog is a list of work items, typically described concisely and recorded in an issue tracker.
Work items can be taken off the backlog and placed on a planning or tracking board when they are selected to be worked on by the team.
The backlog is typically maintained by a product owner. The backlog is an important instrument for communication with external stakeholders on one hand, and team members on the other. Also, the backlog is used to record agreements and decisions regarding the content of each work item, its priority, estimated required effort, etc.