It is an RS232 modem eliminator
(modems allow computers to communicate with one another over telephone lines), that permits communication over a limited distance without a pair of synchronised modems.
A new Single User Remote Connection provides connection to an IBM midrange host over a serial RS232 line, allowing synchronous connection directly with a modem eliminator
or via a pair of modems with a SDLC or X.
The interfaces are usually RS-232, so modems or modem eliminators
are needed for distances greater than the standard 50 feet.