Agile teams use comparative estimates or relative sizing, which uses a relative unit called story points, to estimate more accurately. Rather than trying to predict absolute time frames, estimating in terms of relative size (as opposed
to absolute) allows the estimates to be made more usefully. A team can only finish so many user stories in an iteration. The other responses are incorrect: user stories are not mapped to days of an iteration; estimates are made by
the relative size of user stories, not by hours; value-based prioritization is how the product backlog is organized, not how user stories are estimated.