KH/carbonate hardness, what does it mean for your marine aquarium hobby.

The ways in which some folks discuss this can be as Alkalinity and or acid binding capacity!
When folks are referring to KH, they are referring to the amount of carbonates freely available in the aquariums waters and the most common are calcite and calcium carbonate, both are originally derived from lime stone and become part the essential make up of corals and molluscs shells, etc ,etc.
What’s important here is that when there are abundant carbonates in your aquariums water, these enable the PH to hover at suitable levels by defusing detrimental acids in the water.
The original lime that we usually refer to as calcium in reef aquariums that is a positive ion, its most important use is to help stop the PH acid-base make from closing!
That space between the acid and base that we measure as PH, if this begins to close out of control, the PH drops and all marine life suffers by what I suppose you could label as trying to exist in a totally static electricity environment and oxygen that sustains all life is becoming unavailable in this environment.
What affects KH.
Any and all acidic reactions in your water will use up carbonates and the most common acidic response in your waters is the nitrogen cycling bacteria, if they are very active like during the maturing of your bio media or if you move live rock or if you over feed or something large has died, this will lower KH.
Another common acidic response is when you suddenly change the water chemistry by adding one water make up to another and the most common is a water change!
Just like in our pets gills, if there is a sudden change, by this I mean if you take those gills from one water to another suddenly, the acid response can be devastating, but the aquariums water will recover, your fish may never!
The most common disrupters are from any of the nitrogen cycles biological functions occurring in your waters and also if your fish room is sealed up or your house is heated can put a little extra co2 into the water, all these lower KH!
You see water naturally absorbs all gases in the room at the time and co2 uses up valuable carbonates.
Any and all photosynthetic life including all corals and the main photosynthesises in your aquarium being cyanobacteria and all algae species; these literally suck the carbonates out of your water for varied reasons.
What is a good KH level and keeping KH reasonable!    
The oceans KH is around 7, so we try to keep out marine aquariums somewhere from that figure up 10 and a max of 14, 9 being the best.
How to help KH, just add an alkaline buffer if KH drops and test to get a feel for how fast or slow your aquarium uses up carbonates and from then on your tank life will be fine.
If something is out of the ordinary like water changes or an in tank disruption occurs lowering KH, apply a little extra calcium carbonate along with Epsom salts-baking soda or any form of magnesium really and just to be sure, lock up a little of the main culprit here being acidity by applying a little ammo lock to the water.