A backlog is a list of work items or tasks which are to be finished in order to consider a project or a release completed. It serves as a todo list or queue of items.
The items with the highest priority are at the top of the backlog.
If the backlog item is too large (it is difficult to estimate it or the estimation is too high), the backlog item should be broken down into multiple smaller backlog items.
From time to time it is important to go through the backlog, review whether it contains appropriate items and reprioritize it to reflect changes in external conditions and the current state of a project. This process is called backlog grooming.