What do the numbers in the burndown represent?
The numbers represent the amount of work (number of tasks or sum of estimates) left at the end of any given day. This includes work that is in progress and the work that is not started. 

The first data point shows the amount of work left at the end of the first day. So it doesn't contain work completed on the first day or work that is added to the sprint later.

How do you decide when a task is completed?
A task is considered completed once the task is moved to a state that is mapped to "Done" in the workflow mapping.

Can I specify my timezone and/or working hours?
If you don't work on weekends, you may want to omit those days from the burdown. You can set your timezone and the exact working hours at the account settings.

Do you support time estimates like story points?
Yes. You can use either estimates or task counts as the unit. You can select the unit in the chart settings when starting a new sprint. Learn more about assigning estimates to tasks. 

How is ahead / behind target calculated?
The current sprint view only looks at the tasks in the current iteration. It calculates a velocity for the iteration (number of points completed divided by the number of days passed in the iteration) and then extrapolates how many days it would take to complete the remaining points with that velocity. The behind target / ahead target value shown is the difference between that extrapolation and the actual number of days left in the sprint.

How does the chart handle work estimated vs work done?
The burndown line always shows the amount of work left. If no actual work done value is given for a task, the whole estimated value remains until the task is completed. If the work done is specified then it's subtracted from the estimate to get the work left for that task.

When the sprint is over,  the total amount of work done is calculated for that sprint. Actual work done values are used for the tasks that have them, otherwise the estimate values are used.