kH, carbonate hardness, and total alkalinity, are terms used to describe the buffering capacity of your water. The kH must be maintained at certain levels to keep your pH stable and in the proper range. kH should also be checked periodically as it diminishes over time and must be replenished with baking soda. See the article The Science of Water for more details about kH.
Aquarium Pharmaceuticals makes this kit extremely accurate and simple to use, but their directions for use are very confusing. Also see the article linked above for clarification of API directions.