It's just a baggie of coraline scrapings. The theory is that the coraline will become stressed during shipping and sporulate. When you pour the water into your tank you just get a big boost of coralline spore. If the environment is right it will grow wherever it settles.
I sent out a bunch of these baggies a few years ago and the anecdotal evidence is positive. I haven't done a scientific experiment though.
Shipping starts at $4.05 and depends on your address (USPS Priority).