Click on the task to open its Task Properties.
You can rate the task on a scale of 0-5 points based on its complexity. Click on the drop-down arrow next to Task Points section in Task Properties to assign points to the task.
You will see an aggregate of all the task points in a tasklist at the top.
If you wish to hide the task points, access the Project Settings by clicking on the gear icon, then click on the toggle button next to Task Points
Note: Currently, the points range from 0-5 only.