Su means vinegar in Japanese, and this product is sushisu, rice wine vinegar with salt and sugar added. It's what holds sushi rice together (Japanese rice is naturally stickier than long grain rice, but not sticky enough), and gives it a subtly sweet-tart flavour and a glossy finish. It can also be used in salad dressings, dips, and so on, but go easy on other sweet or salty ingredients like mirin or soy.