CaCo3 is Calcium Carbonate which is found in rocks, hard water etc. PPM is referring to parts per million, so when talking about hardness of water you would refer to the parts per million of CaCo3 in H2O. Calcium Carbonate is also know as Ag Lime which can be added to soil of it's too acidic.

I need to explain this unit to a mechanical guy and please simply this as soon as possible?

Please note that calcium carbonate is written as , note the capital O, otherwise, this will mean a compound containing one atom of Calcium and three mole sof Cobalt.

When you say that some water has a certain amount of ppm of CaCO3, this is a measure of the concentration of CaCO3 in the water. The higher the concentration of CaCO3, the harder the water. Ppm is a very small measure of concentration, of the order of 1 ppm = 1 gram in 1000 litres of water.

Just to elaborate on the answers that SkyRoller and Unky gave, PPM works just like percent (%), except that it means the number of parts per million instead of the parts per hundred.

If you have 20 marbles, and 11 of them are red, you can say that the red portion is 11/20=0.55 or (11/20)*100=55% or (11/20)*1000000=550000 ppm. Those are three different ways of saying the same thing.

Obviously it's kind of silly to use PPM when talking about a relatively large number like 0.55. It's usually used when talking about much smaller numbers. For example, if you have 1000 g of water, and it contains 0.22 grams of CaCO3, the concentration can be written as 0.22/1000 = 0.00022 or (0.22/1000)*100 = 0.022 percent or (0.22/1000)*1000000 = 220 PPM.

Hopefully that's simple enough that even a mechanical guy can understand it. :)

PPQ may throw him a little(Parts Per Quadrillion):
http://en.wikipedia.org/wiki/Parts-per_notation

Another facet of the answer is that the mineral concentration is being measured by precipitating the cations by forming their insoluble carbonate salts. The total mass of precipitated material is weighted and then assayed AS IF CALCIUM WERE THE ONLY CATION PRESENT. No attempt is made to make any further analysis of the cations. So the "as CaCO3" is a little looser than its literal reading.