The Backog represents all the tasks you have stored but haven't committed to do yet. They are ideas, inspirations, activities that don't have a specific date, and aren't in your Plan of the Week.
In Week Plan, you can slice your Backlog by Objectives, Roles, or arbitrary Lists.
For example, when you set an objective for the quarter, you have a list of things you could be doing to make progress for this objective. You can store this list in the Backlog by Objectives.
Week Plan also encourages you to think about your Roles separately. You are not just about work and "life", you may be a friend, a family member, a community member, etc... and each Role may have their own list of uncommitted tasks.
And finally, you may choose to store some tasks in some grouping of your choosing, for example, you want to store all the books you want to read together, this is what the Boards and Lists are for.
Ideally, you want to make sure the Backlog lists are prioritized, the top tasks should be the ones to pick next.
Regularly, maybe once a month, you can do a Backlog pruning activity that will remove obsolete items from your Backlog, and reprioritize the remaining tasks.