These are indicators!
The yellow aura will appear whenever you have to choose a target for a Spell or attack. If a Unit doesn’t have a yellow aura surrounding it, that means it isn’t a valid target for what you are trying to do.
The green aura shows which cards you have that are currently usable . If a Unit or Hero is in play and has a green aura, it is ready to attack. Units and Spells with a green aura (either in your hand or attached to a Unit) can be played, meaning that you have the necessary Mana to cast the Spell, and there is a valid target for its effect (if it requires one).
The skull is an indication that the target unit will die due to the effect or attack being used. This is handy when there are lots of factors involved, such as Armor.