What exactly does it mean if a car's engine has low compression in one of the cylinders?

I don't know a whole lot about cars and engines. I know a little bit. I do know that if an engine has low or no compression in any cylinder of its motor, that that is major bad news. But since I am not an expert, I don't know exactly what might be CAUSING the low or zero compression. Would this indicate... show more I don't know a whole lot about cars and engines. I know a little bit. I do know that if an engine has low or no compression in any cylinder of its motor, that that is major bad news. But since I am not an expert, I don't know exactly what might be CAUSING the low or zero compression.

Would this indicate that the piston has shattered in the cylinder? Does it mean that there is a problem with the engine valves? If one cylinder has low or zero compression, it does mean that the entire engine must be rebuilt, correct?
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