Shotwell lets you flag photos. When a photo is flagged, a small flag icon appears in its upper right corner. You can select the Flagged item in the sidebar to see all photos which have been flagged.
Flagging a photo marks it as special. You can interpret this in any way you like. For example, you might flag all photos which need visual adjustment, or all photos which you want to share with a friend.
Flagging is also useful because you can operate on all flagged photos as a set. For example, you can select the Flagged view and then upload all flagged photos to a publishing service.
To flag or unflag a photo, right-click the photo and choose Flag or Unflag from the context menu. Or use the Ctrl+G or / shortcut keys.