Not a dumb question at all. It can be very confusing!! So the Duchy of Cornwall is a title reserved for the heir apparent and son of the monarch. It’s one of only two titles that comes automatically with a financial portfolio largely from property and land. That income provides funding for the activities of the Duke and their family. The Duke manages that portfolio. So William attending meetings for the Duchy before he’s actually the Duke shows that he’s preparing for the role. It’s part of the transition process that will happen when the Queen passes. It’s basically like Charles preparing to hand over the torch to William. And the fact William attended the meeting on his own- not as like a shadow to Charles- is also significant as it’s something he’s doing as part of his own development now. They’re including him as an important figure in his own right.