In the music gear world it can refer to an effects loop. In that case you basically run two signals from a guitar into an amplifier. One signal could be a "series loop" where the guitar signal runs through the amp and all added effects at the same time. Another signal could be a "parallel loop" where the guitar signal gets split and you can have a "dry" signal untouched by the effects plus a "wet" signal with the effects. I'm really oversimplifying but I wonder if there is something similar in your field where this could have a similar application.