This "daisy-chain" cable allows for powering multiple BOSS compact pedals from a single pedal like the LS-2, TU-2, or NS-2.
Boss is a manufacturer of effects pedals for electric guitar and bass guitar. It is a division of the Roland Corporation, a Japanese manufacturer that specializes in musical equipment and accessories.
For many years Boss has manufactured a wide range of products related to effects processing for guitars. It includes "compact" and "twin" effects pedals, multi-effect pedals, electronic tuners and pedal boards. In more recent times, Boss expanded their product range by including digital studios, rhythm machines, samplers and other electronic music equipment.
They also are now manufacturing solid-state amplifiers and speaker heads such as the Waza and the Katana. Both feature multi-effects units meant to emulate Boss' classic effects pedals.