The Burndown chart is a graphical representation of work left over a period of time for a particular Sprint. The y axis in the chart contains the work left in team measurement units which can vary from team to team, and the x axis is the time in days. The following screenshot shows you a Burndown chart sample:
The y axis represents the measurement units of work which can be Story points, business value, or hours. The x axis in the chart represents the Sprint timeline in days.
During the start of the Sprint, all the work is in the "to do" state. As the Sprint progresses, the tasks are completed and the work remaining decreases over time. The Ideal graph is a guideline representing the ideal burn over time. The Actual graph represents the work burn over time by the team.