Rooms and Maps are the foundation of your games. They provide the "places" that you and your players can go, and each game can have as many as you like.
Rooms are areas you can hang out and video chat, and store materials your players can look at, read, listen to, and watch. You use large .jpg images as room backgrounds, and the backgrounds always stretch-to-fit each player's screen size. Think of rooms as usually being in the first-person perspective.
Maps are like rooms in that they are also areas you can go to communicate, however they are the surfaces on which you play your game. Maps are usually top-down perspective, much larger than your screen size, and can be dragged (panned) and zoomed in and out. The map is your tabletop or your game board. You can add square or hex grids, fog of war, and of course tokens to your maps. A map can be anything from a map of a continent or town, to a detailed grid map of a dungeon level. It can even be a chess board if you like.