Blanching is a process in which you boil or steam vegetables briefly until they are partially cooked. It is an essential step before freezing many vegetables including broccoli, leafy greens, string beans, okra, and asparagus.
Vegetables that are frozen without having been blanched are safe to eat but have "off" colors, textures and flavors. If you've ever stuck a bunch of raw spinach into the freezer and taken it out later to find you had a darkened, gooey mess, you understand. Blanching stops the enzymatic activity that decays vegetables. These enzymes can survive freezing temperatures and continue the decaying process even though the food is frozen. Pre-treating the food in boiling water or steam kills the enzymes.