They are very simple, imaginary machines. They only have one general purpose register, a single non-addressable register to which you can copy or swap the value

They are very simple, imaginary machines. They only have one general purpose register, a single non-addressable register to which you can copy or swap the value in the main register and four I/O registers for communicating with adjacent machines. To get anything complex done, you need to use multiple machines and much of the puzzling is spent figuring out how to divide the responsibilities.