For agile planning to work, you must have a prioritized and sized (estimated) backlog. A prioritized backlog is simply a list of work that needs to get done that is ordered by priority. Essentially, the work at the top of the list is the most important and should be done first. In many cases, customers have a difficult time prioritizing lists of features, however, this makes the prioritized backlog even more important to produce. Another requirement of this backlog is that each of the items must be sized, which is another way of saying that the item should have an estimate.