What does it mean when a motor has a knock?

"Knock", sometimes called "ping" is mostly caused by pre-ignition, a function of improperly set timing. Since the advent of computer-controlled ignition, this once-common problem has greatly diminished. When it happens in modern cars, it can be caused by a buildup of carbon in the cylinder heads, which is usually traced to cheap, bargain-brand (non-labeled) gas. The last car I owned that developed a problem with pre-ignition was a 1980 Chevrolet Chevette, and that was only after it passed 85,000 miles.