Flagging tasks is a great way to highlight and track your important tasks (only you can see your flags.) Just hover over the task you want to flag in the list view and click the flag icon beside it.
You can also flag tasks by clicking the flag icon in the task pane menubar, beside the "Subscribe" and "Delete" icons.
You can view all of your flagged tasks in the "Flagged" list, located in the Favorites menu. You can also focus on the flagged tasks in any list by clicking "Search tasks" at the top of the list to expand the filter bar and reveal the Flag sort icon. Click the flag icon to show only flagged tasks (the flag will turn orange) and click the icon again to turn the filter off when you're finished.