I don't know what the technical name for what it is but is helps strip detergent build-up. You can find it at Walmart. It comes in a little pack that has two plastic capsules in the pack. The capsules look like the ones from a bubble gum machine. Insind the capsules are the powder. You don't find it on a shelf but usually hanging off a shelf in the laundry detergent section of the store. I usually see them where the RIT dyes are.
HTH!