When a child is a ward of the court it means that the child becomes and is subject to the protective supervision of a court and to it's orders pursuant to that supervision of the court.
A child can become ward of the court when the parents or legal guardian are not available for the child to care for them or when the child has been taken from the custody of there parents and put into protective custody with the state.
Also if a Juvenile is arrested and held in a Juvenile detention center then the Juvenile can become a ward of the court and is also subject to the courts protective supervision of the court and also to it's orders pursuant to the supervision of the court.
The child can sometimes still be a ward of the court when they are sent to a foster parents home temporarily until they find a permanent home for the child.
Or until the child goes back to the custody of their original parents.