A burn down chart is a visual representation of the amount of work that still needs to be completed before the end of a project. A burn down chart has a Y axis (work) and an X axis (time). Ideally, the chart illustrates a downward trend as the amount of work still left to do over time "burns down" to zero.
A burn down chart provides both project team members and business owners with a common view of how work is proceeding. The term is often used in agile programming.