It looks like a jack to XLR converter but it really is an active DI box. It will allow you to connect the output from acoustic and electric guitars, guitar amps, keyboard, mixers, tape machines and just about anything else directly to the mixer, without using a microphone, and with no hum loops.
A Direct Injection (or DI) box is a very handy piece of equipment for any public address rig or recording studio, whether for band or general use. It will allow you to connect the output from acoustic and electric guitars, guitar amps, keyboard, mixers, tape machines and just about anything else directly to the mixer, without using a microphone, and with no hum loops. The Active DI will convert unbalanced inputs to balanced, allows the level to be set to something reasonable, and comes in two flavours. Schertler Active DI consists of discret circuitry in class A to deliver a natural and acurate signal to the mixer.