All users (except collaborators) on all account types can change task importance.
Use the importance feature to mark a task with high, normal, or low importance. By default, tasks are created with normal importance and aren't designated with any symbol in Task view or List view. Tasks with high importance are designated with an exclamation point in List view and Task view, and tasks with low importance are designated with a down arrow.
Open the task you'd like to add importance to.
Hover your mouse to the left of the task’s name and click the exclamation point that appears.
Select the importance level you'd like to apply from the drop-down list that appears.
Once you've set importance you can sort tasks by importance.
Additionally, you can right-click a task in List view and hover over Importance. From here you can select importance for the task.