newbie question about screen and entities

I am new to the wonderful playclj, and am confused about [screen entities] as arguments for the functions. I went through the tutorial, but am still not getting it i think what these arguments are and what are they containing? I know screen is a map (a record) containing various functions and entitites should be a vector containing aspects about the objects in the game. What kind of values are the entities and screen holding, are they dependent on the functions we define? I would be very thankful if someone could give me some help