Iteration burn down chart or Sprint burn down chart is used in agile project management. They are used to monitor and control the iteration (sprint), which is a 30 day or lesser bucket of work.
The ‘X’ axis always denote the duration of the sprint. The ‘Y’ axis has the sum of the task estimates. As the iteration progresses, data points are plotted based on the balance effort required to complete the balance activities within the iteration (the red line). The blue line indicates the ideal line, where the balance effort becomes zero on the 20th day (In this project, they have decided to have 20 days iterations / sprints). As long as the red line which indicates the balance effort required to complete the iteration is hovering around the blue line, there is no risk involved.
Though iteration or sprint burn down chart originated from the agile project management, we recommend it in traditional project management as well, especially when approaches major milestones.