Soba Yu is the water that soba noodles have simmered in.
When a customer in a restaurant has finished eating his soba noodles, it’s brought to the table in a small pot.
You pour it into the remaining broth in your noodle bowl to make a “final course” soup.
The simmering water is considered to contain nutritious elements from the noodles that dissolved into the water.
Language Notes
“Yu” means “water” in Japanese.