A common thing agile teams are asked to do is plan, given historical performance, how much work they think they can reasonably commit to in a fixed increment of time. I'm going to tell you that in most cases, this is the most destructive thing you can do to a team, a product and your business.
Why? Because you are passively planning to be mediocre.
Sometimes the capacity and skill set of the team vary drastically from sprint to sprint and even mid-sprint. When that happens you'll want to pay close attention to how the team is doing against the sprint backlog. All else being equal, that's the job of the burndown chart.
The trouble is, it doesn't handle this situation very elegantly at all because it assumes even capacity and that isn't always the case.
Here's a simple way you can get around that problem.