Sprint burndown charts are used to track progress in Sprints. Lets look at a couple sample burndowns.
Sprint Burndown Sample #1
We can see in this example there are several risks. First, we can see by the increase in pending work that the team is either taking on new work or the work they have in place was not estimated/decomposed very well. Next, we can see that the team is taking on more work than they are finishing. This is evident by the increasing work in progress, highlighted in yellow. Last we can see they started off comitting to nearly 300 hours of work and only finished half that. It is the ScrumMasters responsibility to help the team see the issues and get them fixed so the team can get on its way to being high performing.
Lets look at another.
Sprint Burndown Sample #2
in this burndown we can see things are better. The pending work stays relatively flat, the work in progress is small, indicating there was thought put into place on the tasks needed to accomplish the stories, and the task sizes are small. Last we can see the team finished their work. These are typical results after two to three months of a team being together, the results above are typical of teams that have people change every sprint.