Roll 20 is a virtual table top. Without magic (API), it is basically just a blank canvas for you to have little tokens on that you can move around or have your players move around. To be more complex, it allows you to store multiple 'tables' with pretty pictures that can make it look like a legitimate place for your players to pretend like they are at really. This allows you, as the GM, to move players around these 'maps' to further your game. The tokens can store minimal information too. They can have three visible trackers that can be assigned numbers to represent many things, either hp, mp, wounds, stun, ammo, plot points, fate points, health potions, etc etc. These tokens can also store 'attributes' which are more numbers so you can save some information about each character. There is also a chat function that allows everyone to type into it. This chat function also houses a pretty good dice engine for players to roll with and allow the GM to see. Basically, its just a tool, not a game.