Atlas Arcade is a way that players can earn extra Atlas Bucks by playing popular, non-Atlas Earth games. You'll find it as a button near the bottom of the Shop tab.
To use the Arcade, browse the Catalog page until you find a game you'd like to play. You'll be prompted to download the game, and turn on tracking. Then, you can play for set amounts of time or achieve certain in-game objectives to earn Atlas Bucks in Atlas Earth! To find games you've downloaded through Atlas Arcade, click the "My Games" tab. That tab is also where you can check what the objectives are for each game you have downloaded.
Have fun, and enjoy the extra AB!