Kh stands for Carbonate hardness Johnny
This is the measure of calcium and Magnesium saltd that are present in the water.
Eapecially if your water is alkaline, soft water [our area of the UK] tends to be associated with acidic conditons [we use Kusuri Lythaqua to bolster the water], its a form of calcified sea weed that breaks down over time.
Hard water indicates the water is alkaline , if you are experiencing ow leavels of Kh you may need to add minerals to your pond as your koi and goldfish need a certain amount of calcium and minerals for their good health.
It will help buffer the Ph whch will help you maintain a constant ph level .
As I said we use Lythaqua but as @Nephen said earlier she uses crushed oyster shells instead.
Differing medications react differently in soft or hard water areas so its best to know about your water conditions should you suddenly need to treat your koi .
Soft water makes the chemicals more toxic whilst in hard water areas its less toxic .
Hope you understand all this my friend
Dave