[Haskell-beginners] Monad Transformer Type Signatures

I am looking at code from a tutorial that has the following type signature for a function: getScreen :: MonadReader AppConfig m => m Surface AppConfig is a data structure that was defined previously. My question is how am I to interpret this type signature? Is it any m belonging to the MonadReader AppConfig class? or any AppConfig m belonging to the MonadReader class? Since I can't see any instance of the MonadReader Class being defined for MonadReader AppConfig I am puzzled. I would appreciate some clarification or a pointer to a prior discussion of this issue. I have tried to look on my own. Thank you.