It is an MDA/Hercules 720x350 monochrome monitor, usable with the later Tandy 1000 models (RL/SL/TL) when switched into monochrome mode, or with any PC equipped with an MDA or Hercules card.
It was probably more commonly used with the Tandy 1200, the very generic straight-up XT clone they sold for a while. (May also have been paired with the 3000 AT clone.)