In general, if it is an offical Roll20 conversion of a module, great care has been taken to include everything from the print version. Sometimes the print version does not include a map of an area, or vaguely indicate that an encounter might take place in an undescribed location. Example, module writers do not write things with a VTT in mind, and might have an encounter that takes place at an inn, but no inn map is provided in the book. In those cases, Roll20 typically provides one or more generic maps: a pre-built page with a background texture upon which things can be drawn or placed ad hoc. If an NPC is mentioned but there is no art, a text token is usually provided. If there is no stat block, a generic token is provided. Every illustration is included either in the compendium, the adventure handouts, and/or image handouts. Great care goes into making the game require as little prep as possible. As both Aaron and Al have said, though, time spent beforehand familiarizing yourself with where those piece are and how they are used together is invaluable. Roll20 does a lot of cross-linking to facilitate finding things.