Agreeing with neb2, I would add that it order to simply put it in words and grasp it. It is a system that is used to avoid or eliminate undesirable outcomes or events. It is there to make sure we do not end up with any risk. In order words, it eliminates or reduces the probability of a undesired consequence and in return our company, product, market, and customer are satisfied at a maximum.

This is how I understood CAPA and this is how I would explain it to someone who did not have any idea what CAPA is.