Yuzu kosho is a concentrated Japanese paste of chili peppers, yuzu peel and salt, commonly fermented. It adds citrus fragrance, salt and heat to hot pots, grilled foods, soups and dressings. Green and red versions use different stages or colors of chili and citrus.
Use a little at first: it is a seasoning rather than a pourable sauce. It is not ponzu, and its name does not mean it necessarily contains black pepper.
Reference: Further reading.