Blanching means to plunge something (a vegetable in this case) quickly into boiling water and leave it submerged for a short amount of time. Once you remove the vegetable from the water, it needs to be immediately placed into a bowl of ice water, also called an ice bath. For this brussels sprout salad, blanching is needed so the leaves can regain their color and that crisp crunch. It also helps get rid of some of the bitter taste a brussel sprout naturally contains.