When an item’s form is displayed, move the cursor over the gear icon in the top right-hand corner of the form. From the list of items, you can click on the star next to an item to flag or unflag the item.
In the list of items in the library, the flagged items now have an orange star in the second-last column from the right. The orange star is used to flag an item or items that you want to draw special attention to, whether they are favourite items to be reviewed or completed.