This line graph represents the sum of hours that have been estimated on the tasks assigned to your stories in the Sprint. The ScrumMaster will choose a time of the day (as long as it’s the same everyday) and will plot the current figure on the chart. A common burn down will look something like this.
The red line represents the actual work left to complete, where the green line is showing the ideal trend line. If the red line is above the green at any point then the team is not moving at a fast enough velocity.