Is valid what does it means? How to use it ?and when to use it?

[TVinforest]

So for example you have Player Inventory and Some AI NPC trying to steal something from player. So the function will be like: get array of items from Inventory, sort them and get most valuable item. What if player inventory is empty -then the array is empty and function can’t return valid state. Or NPC have reference in Blackboard to his TargetToFollow Actor and then this Actor was destroyed so the reference variable will be no longer valid for the next update.