Calcium water is a mixture that helps the Pomona's pectin gel without the tons of sugar required in conventional pectin brands. It comes in a little packet inside the Pomona's box. You simply mix the contents of the packet with the required amount of water. It stores in the refrigerator for a number of months. But with all the jam you could make this summer, it probably will get used up before it expires!
Now, on to sing the praises of Pomona's Pectin...
It is made of 100% citrus pectin, derived from that bitter white pith stuff under the peel. Many fruits have natural pectin (apples are also high in it). Some fruits are very low in pectin (peaches, apricots), and as such, require more to gel the jam. In other brands of pectin, copious amounts of sugar are also required to set the gel. I recently read a recipe calling for as much as 7 cups of sugar for 5 cups of mashed apricots. With a Pomona's recipe, as little as 3/4 cup of sugar is required for 4 cups of fruit. You can also use 1/2 cup of honey or other alternative sweeteners such as stevia.
Whatever brand of pectin you choose, please follow the directions in the box exactly, or you may end up with runny jam suitable only for ice cream topping (not a bad thing, though).
It's chemistry, people ... and you thought you'd never use it after high school!