Flagging tasks is a great way of staying on top of important tasks as your list of things to do grows. Flagged tasks are filed with normal tasks, and are also quick to find on a separate page that only shows your flagged tasks. Plus, they’re easier to spot when you’re looking through your lists to see what you need to do next.
To add a flag to a task, just hover your mouse over the right side of a task in your Inbox or any of your lists, and click the grey flag. Then, if you want to remove a flag from a task, just click the flag again.