Uncertainty in the Product Backlog is a big risk for the schedule of a Scrum project. The problem is that the full scope of the release can be quite hard to estimate because the requirements are not well-know early on. Confounding this problem is that frequently the release date is a hard deadline. This means that an Agile team must perform an unknown about of work in a fixed amount of time. The “Cone of Uncertainty” describes the reduction of the uncertainty about scope after each iteration. At the uncertainty is eliminated and the exact amount of scope is known.
In anticipation of scope problems, the software development schedule should be aligned to address the highest priority feature sets and features in early iterations, and defer lower priority feature sets and features to later iterations. To reduce the Cone of Uncertainty and the risks around missing a release deadline further, the requirements lead (product owner) and the Scrum team should actively defer low priority features from all feature sets to later sprints. This avoids the case of being unable to release a minimally acceptable product because time was spent on low priority user stories / features from high priority feature sets when it could have been spent on all individually high priority features across feature sets. Applying Agile and Scrum properly benefits organizations by helping them to address the cone of uncertainty and reducing uncertainty, empowering software development teams to deliver business value. With Agile, the estimation error normally goes down iteration after iteration (again, if the project is well managed).
More references on the Cone of Uncertainty in Project Management & Scrum