Assigning estimates to tasks
A time estimate such as story point can be associated with a task by inserting a number in the parenthesis as a prefix to the task name.
(6) Task name
Means that the task is estimated to be size of 6. You can also use decimals e.g.
(6.5) Task name
The default value is zero if no estimate is given. Optionally, you can record effort already spent on a task by putting it in square brackets
(6) Task name 
Now the task has 2/6 units completed, leaving 4 units to be done.
Once you have estimates in place, you can choose to use estimates instead of task count as the burn unit in the chart settings.
In Sprint Burndown chart, whenever you update the number in the square brackets, the burndown chart is updated accordingly. A task is considered completed once it's moved to a list that is mapped to the done state, and the number in the square brackets is recorded as the points done.