kH stands for the carbonate buffering ability of the water. The best way I can describe it is like a car's shocks. IF you have good shocks, when you go over a bumpy road, the car barely bounces. If you have bad shocks or no shocks, when you go over the bumps, the car bounces up and down and up and down.
kH in a tank are the "shocks" for the pH. With a good kH, it is hard for the pH to vary. With a low kH, the pH can easily be affected by many things.
Yet another thing to buy, huh? It is kind of important to know your kH. Perhaps if you took a water sample to a fish store and asked for them to test it. Many will test for free - just ask for a numeric reading on kH.
You can raise your kH (and pH) cheaply and easily using just baking soda. If this is needed, advice can be given.