Adaptater to listen passively to a RS232 communication link.
That little box is an adapter for RS232 communication analysis using a Yocto-RS232. If features two DB9 connectors, one male and one female, connected directly one to the other, that can easily be inserted on a serial cable. The RJ12 connector will allow to connect it to a Yocto-RS232 using the RJ12 (6P6C) cable provided, so that the device can passively listen to the RS232 traffic.
With that adapter, your Yocto-RS232 will be able to transcript the detail of RS232 communication, including the direction in which bytes have been sent. The adapter is works like a straight cable (passive), it won't interfer with RS232 electric signals.