After proper application, the marks convey your mental state for the next 8 hours. A creature that can see you and makes a successful DC 10 Wisdom (Insight) check can discern whether you are happy, sad, angry, disgusted, surprised, or afraid, as well as the main source of that emotion.
For example, you might communicate fear caused by a monster you just saw around the corner, grief at the loss of a friend, or happiness derived from pride in your performance in combat.
A Darkling Elder has advantage on this check.