KH is the measurement of your water's ability to maintain its current PH. The higher the number the harder it will be to change the level of your PH.
If PH is not a concern for your fish I would leave KH alone. However if you wish to measure it you will require a KH testing kit, these are normally incorporated in the GH testing kits but do check to ensure GH and KH are present before buying.
KH (carbon hardness) is measured in mg/L.