What does "possession is nine points of the law" mean?

Actually holding something is better than merely claiming it. For example, When Karen told John he must return the sofa he'd borrowed, he said possession is nine points of the law. This term originally alluded to nine elements that would aid someone's lawsuit, among them a good lawyer, good witnesses, a good jury, a good judge, and good luck. In time, however, the term was used more for squatter's rights. [Late 1500s]