A Ward of the court means that the person who is made a ward of the court is someone who is under the protection of the court.
When you become a ward of the court you are usually appointed a guardian by the court.
When a minor becomes a ward of the court it's usually because of something serious and means that the courts will take primary responsibility for the control and treatment of a minor.
So basically the child is likely gonna be in court custody and be placed in foster care or be in a youth shelter until things get resolved.
Sometimes when there's a custody issue such as both parents are not treating the child right and abusing the child the court will then make the child a ward of the court.
Or if the child has run away and then has been picked up and found by police the child may sometimes temporarily be made a ward of the court until the parents are found.