What exactly is meant by coronary artery spasms?

Coronary spasm. Coronary spasm is basically the artery constricting or clamping down. The coronary artery has a middle layer of muscle fibers that if triggered will constrict. It can occur because of smoking or sometimes randomly. It is most common in women.
Heart strain. Coronary artery supplies blood to your heart muscle. Artery spasm changes the lumen within so blood flow is compromised. During arterial spasm, the flow may be significantly reduced as to cause damage to your heart muscle - sometimes irreversible aka major heart attack.