Atari800 is an emulator of Atari 8-bit computer systems including the 400, 800, 1200XL, 600XL, 800XL, 65XE, 130XE, 800XE and the XE Game System, and also of the Atari 5200 SuperSystem console.
The Atari Operating System ROMs are not available with this package, due to copyright. You'll have to either make copies of them from an old Atari computer, or see README.Debian for other ways to obtain them.