'Pan-dressed' simply means that the fish has been trimmed, scaled, gutted, boned or filleted and is ready to go straight into the pan in the recipe of your choice. This type of preparation is common in quality fish shops and fish markets. If you choose a whole fish, most fish shops will do all of these things for you, and should do them free of charge, but they may need notice at busy times. And you might need to explain your recipe because certain dishes required skinning as well as filleting.
If you ask your retailer to bone a fish, remember to ask for the bones to use for stock as these will not always be included automatically.