Ooh, you guys. Kombu is a type of seaweed that's super-high in umami. You've very likely benefitted from it many times without even knowing it, since it's one of the main ingredients in dashi, the stock used to make miso soup. Placing a little piece of kombu into cooking water adds a subtle umami flavor that makes sushi all the better. It's optional when making sushi rice, but if you have some, I recommend using it.