Any Entity in the game can be either Dimmed or Undimmed. At the start of every round all the Entities Undim.
When a creature moves or attacks it becomes dim. It can't attack or move if it is dim. Dim/Undim is a way to make sure your Figures only take one action a round.
This pulsating blue circle indicates that your Craig Scout is undimmed so we can still move or attack with him. The Shield Warrior next to him is dimmed so he can't move or attack till next round.
There are also some special actions that will cause Entities to dim. If an action causes an Entity to dim you will see this icon on the card description. Here is an example.